I think the -12dB is more for convenience, and being able to "set it and forget it".
I am not aware that anyone has ever posted a comparison or analysis to demonstrate the existance of a sweet spot, or even a problem spot, in regard to m10 levels, or gain knob setting (whether set to reduce levels or increase levels). Is there a point where a high average RMS causes a degradation of capture quality with the m10? I am not aware of one, but it is a good question. I believe most folks mentioning a -12 dB peak level are doing so to leave headroom for surprises in the source material.
With the r09, there was most definitely distortion when the trim/gain setting was at or below 8, especially when average RMS levels were high (like from heavy bass). It was as if the recorder was internally brickwalling. That aside, the r09 sounds great.
A blind comp comparing the m10 when run at -12 peaks vs. say -2 peaks would be interesting. But only with the right source material.
I personally think that peaking at -12 is a bit low. But that is situation and source material dependent. If I am recording a very quiet source, I probably want more gain from my front-end preamp, and want the peaks higher. That will also increase the average RMS levels, which tend to be low with those sources. If I am in a situation where I cannot ever monitor or adjust levels, then that would be another reason to run the levels lower. For a loud PA source, that also has a higher average RMS, a lower peak is fine.