Well interesting twist of the original thread.
Actually if I call myself a plain user (no problem to do that), I would be one of the less uncomfortable (or more comfortable, to eliminated the double negative) to do compiles myself. Check my registration date here. I was a dev (not for MAME) in my early days and after all I started when computers were 8 bit and one of my vacation hobbies (ok just during evening sleep of the rest of the family - I am not THAT a geek) was to write BASIC ON PAPER (no portables back then). Interestingly enough most didn't even need debugging back when typed on the computer. Anyway... point is that even for me, what you suggest is not a realistic solution.
Most plain users even with a friendly GUI will not even touch something that needs re-compile for every update. 99.5% of them will prefer the drop-in-place exe. Imagine if all software was like that! For people coming from Linux of course this way is more common (BUT REMEMBER BUILD ENVIRONMENT IS ALREADY PART OF THE OS SETUP) still even there, it is a dying trend. Even if it becomes as easy as installing Office (you see I make it easier than installing Windows that you say), I don't think we could convince anyone reinstall Office with the rate MAME builds come out. It is unrealistic.
Personally I would prefer a 10% or 20% slow down of betas, if official merged build was there. (everybody talks for himself of course)
Only way I would "stomach" a user-re-compile for every update would be if it was a transparent part of a very smart installer. (which btw should also support UNC paths as my whole emulation is on a huge share :P) I think this is a big project BY ITSELF. (much simpler than just providing universal-all-enabled exe I think)