"Repeat a note" by clicking a notehead then pressing "R"

This has come up before. It applies to regular notation.
In Sibelius, regardless of whether you are in note input or not, if you select something and hit "R" that item is repeated. In MuseScore, you have to be in note input mode for "r" to work. An extra click.
If you are used to working this way, it seems faster to, for example, be adding notes in one staff, move to another staff to do something (not adding notes) come back, select the last note, hit "r" as needed to repeat it. You've added notes without being in note mode. Now you can move those added notes to the needed pitch. Adding note first, before worrying about pitch can be useful sometimes.

This is also useful when at the end of a piece you decide you need some extra measures. Rather than ALT+SHIFT+B/ input # of measures/OK, You select the a blank last measure/ hit R number of times.

I know it seems like the difference is only a click or two, so why bother. But then there is having to stop an remember the differences. And for someone like myself who uses the mouse most of the time, the less shortcuts I have to remember, the better.

Again, what would help the most is an actual real world score and a specific example that comes up where repeating a single note in a measure you've already entered is something that comes up often, and then you can explain exactly what the semantics should be of that one operation in that one case. Then we can see how it can be generalized. I'm still having trouble imagining cases where repeating one note and then going on to do something completely unrelated - not simply going into note input mode and entering the note normally, so you can then continue entering more notes - would come up more than once a decade or so (I've used a decade and it hasn't happened to me yet, and I've composed hundreds of pieces in MuseScore). This is again why I keep saying, please please please please please provide a specific real world score and talk about a specific note of a specific measure so we can understand the context better.

Again, I'm not saying the feature would be useless, I'm trying to understand the real world case case to help prioritize the feature and to make sure any proposed design implementation actually solves the problem. Or it might turn out that once we understand the real world use case, we'll see a different solution solves the problem better.

One thing I think I'm getting what you are saying, for example, is that you don't actually want a whole bunch of repeated notes - you are planning to change the pitch of the note. You are simply using "R" as a quick an dirty hack to avoid the need to go into note input mode for some reason. That is, rather you actually have a series of notes you want to enter, but rather than use note input mode normally, you are choosing to enter the wrong pitches using R and then change them using the mouse. Well, maybe really says, it would be useful to be able to place notes directly with the mouse without even needing to resort to the kludge of pressing "R" first (after all, you say you want to avoid the keyboard). Like, what if you could, say, Alt+click while in normal mode to place a note? Or, what is Ctrl+Shift+drag on a note allowed you drop a copy of the note anywhere, just like it does for text and other elements? This also comes up in thinking about how the MuseScore interface could be optimized for touch devices. This could open up a world of possibilities, not just for the few people who are using "R" in the unusual way that you are, but for tons of people.

The point is, let's talk about actual real world problems and the best possible ways of solving them, rather than focusing immediately on one possible solution that might not end up being ideal. Maybe the end result is both feature get implemented - the narrow one useful to only to the one or two people who initially suggested the idea but also the broader feature that could make a real difference in overall usability for everyone.

BTW, I do recall you mentioning you were having trouble using the drum palette, but I don't recall any clear explanation of what problem you were perceiving there either. So maybe start a new thread, attach a specific score, and again, describe one specific task you are having trouble accomplishing, and we can help.

Be out of note entry mode, select your last 1/8th with Shift+click, then:
R, R, R, R, X, ←, ←, ←, ←, +

I will preface this by saying I see no reason that someone with sufficient time on their hands shouldn't implement your suggestion. Makes perfect sense to extend the command this way and doesn't hurt a thing, even if it's hard for me to imagine this exact scenario coming up often enough for the single saved click (no need for Shift when selecting the "D") to seem worth worrying about.

That said, and since you specifically asked:

For me personally, if I wanted the repeated notes, most likely I'd have entered them to begin with while still in note input mode. Not that I never change my mind about notes I've already entered, but when I do, I typically just go back to note input and enter the new notes. That's true whether it just so happens to be the same note repeated or - as is surely a thousand times more likely - it is different pitches and/or different rhythms I want to enter. That is, I wouldn't tend to think to myself, "hmm, I've got some notes to enter, they happen to be the same pitch and duration, I wonder if there is a way to do it without actually using note input mode like I would for every other edit I might make".

So, if this did come up, what I'd do is the exact same thing I would do if the notes I wanted to enter were anything else - four full repeated chords, four different eighth notes, or two quarters, or whatever. That is, I'd click the existing "D", press N to enter note input, then enter my new notes: " + D D D". If it was a full chord I wanted to repeat, it would be "+ R R R". If I wanted four different eighth notes, it might be "+ E F G" or "D C B A". if I wanted two quarters, its might be "5 E F". I just can't see myself trying to finding some special trick to avoid note input mode just for that one case where the new notes I want to enter happen to be the same pitch. "+ D D D" is four keystrokes, it's hard to imagine improving on that for entering four notes.

But FWIW, if for whatever reason going into note input mode just wasn't an option, I'd Shift+click the existing "D" to range-select it, then press "R R R R" , then go back add the tie. This would seem awkward to me, though. It's more work than going to note input mode because you have to add the tie separately, and it only works for repeated notes.

Anyhow, again, it's certainly harmless enough to also add code to allow "R" to work with a single note selected in normal mode, so I do hope you do go ahead and add the Suggestion :-)

I love that this was added! It's a very important part of my Sibelius workflow, it's such a freedom to have multiple ways of entering the notes you want. Since I'm one of the people who uses this feature a lot, I thought I'd give some feedback.
Something I don't seem to be able to do is to change the note value of the last note I entered, and then keep on repeating notes right after. What happens is it exits Note Input mode as soon as I change the rhythm. This feature would be perfect if it kept the current notehead highlighted after changing the rhythm so that the user can keep pressing R to repeat more notes. I think this is how many Sibelius users will expect this feature to behave, and it's one of the reasons why I think this is such a fast way to enter melodic lines once one gets used to it, especially when composing. Perhaps there is a way to do it that I don't know about though, I should add that I've only recently started using MuseScore.