I think one of the reasons Mono rocks so much is because of the scoring system. It's not just some arbitrary number, there's a definite limit, and you get the graphical representation as well. It's so much more tangible than just a higher number.
Yeah that's a good point.....76% means a whole lot more to people than something like 1,692,500 does.
Also the scoring system is obvious. Everyone knows exactly what they have to do to get a good score unlike many shmups with fancy score multipliers etc.
I'm not against more complicated score systems with multipliers and combos but the simplicity of mono is probably why it appeals to many non-shmup fans......which it does as I have a whole bunch of emails saying so.
Having said all that, it probably doesn't appeal to hardcore shmup fans for exactly those same reasons. It's a casual-shmup