The more research I've done and information I've gathered: MP0 just seems like the best. It's the best for XP for paragon. It's the best for legendary farming. It makes me sad. Why build such a great end game system of MP (and a fun one) and not reward you really for challenging yourself.
There's no way in hell a 111k DPS character's most efficient MP level for paragon XP is MP0.
Most of my characters ( all beside my DH ) have between 30k and 60k DPS and my optimal MP is between MP1 and MP3.
The issue is that gearing for higher MP's is a complete waste of time. You always gain more xp and loot per hour at the lower MP's because the XP bonus and rewards do not scale appropriately with monster HP with each Monster Power. Monster Power 10 for example will NEVER be a good place to farm.
You are right about the higher MP levels.
But now about the lower.
If you have decent gear, it's better to play, say, MP3 than MP0.
If you have godly gear, it's probably better to play MP5-6 than MP3.
I don't think many players ( if any ) would benefit from playing higher than that, and most likely not MP10.
But the game isn't done yet, think about it.
What about when they release the expansion and we have more powerfull uniques?
What about when they buff some skills?
What if at some point they increase our level from 60 to 70, or even 100?
So far, MP10 isn't of much use for paragon leveling/MF'ing, correct.
But it's usefull for other things ( keyfarming, ubers ), and the little bit of MF these level gives you are just an added bonus.
But for the regular player ( who can't go through MP10 at all ), the lower levels ( MP3-MP5 ) are still good XP, better than MP0, and you can go higher and higher as you progress.
It's easy to notice with an high DPS character.
My highest DPS character is my DH ( with sharpshooter ), like 200k or whatever... And clearing an act on MP1 takes pretty much the same time as MP0. The trash dies in 1 hit on either level, and the elites dies in 8 or 10 seconds on either level.
So it's just a straight XP/MF/GF boost with no added difficulty/time.
If you have 500k DPS, you can probably say the same about MP5.