Some healthy discussion here, so let me preface this by saying I appreciate all of you working to keep the default bindings useful and not excessive.
That said, I still feel there should be a single binding for (yank-pop -1). Here's why:
First, all of the other commands that traverse something (that I can think of) have an opposite bound to an equally convenient combination.
C-n/C-p and C-f/C-b, C-v/M-v, C-s/C-r are some examples off the top of my head.
Of course 'M-- M-y' works, but that's like saying 'M-- C-n' does the same thing as C-p.
(As a side note, I'll admit I didn't actually know/remember M-- is a shorthand for 'C-u -1'. That does make the common case of overshooting the item you want by 1 a lot less painful.)
Second, I fully agree with Clément that just because a command is trivial, it's automatically unhelpful to bind it. Again, see C-n/C-p.
And lastly, if a user has a custom binding for M-Y or C-M-y (I'm not dead set on either), that will still prevail.
I don't see why they'd be any more reluctant to override those than M-y, which you said lots of people do.
It's pretty hard to google "M-y", much less "M-Y", without Google deciding I'm actually looking for "my", so I'm going to trust you on that.
So that's my 2 Cents. If you're still unconvinced, feel free to close this :)