The goal of Monster Power is to give players the ability to scale up how challenging enemies are in each difficulty and, in return, get some pretty cool bonuses to adventure stats as well as the opportunity to earn bonus items. Different players have different play styles, though, so we added 10 power settings so you could decide what challenge level is best for you. For some, MP5 might be best. For others, MP1 or MP2 feels right. If you’ve got a lot of supporting gear, maybe MP7 or MP8 is your so-called “sweet spot.”
What we didn’t want to do, however, is make players feel like they absolutely needed to play at the higher MP levels in order to get the best rewards. This is one of reasons that we didn’t add in Achievements for Monster Power, too. We wanted the primary motivation to be the challenge and finding a level that you enjoy. I gather from your post that you feel the bonuses given for challenging yourself aren’t really worth the effort, but my question is what would you change about it? (Keeping in mind that we don’t want to force players into the higher MP levels.)
Like many others have probably already said, the issue with the MP system is the scaling just doesn't justify the higher levels.
For example, I easily farm MP6 - that's my "sweet spot" as you put it, in terms of speed of kill as a function of speed of movement across the zone.
I used to farm MP8 however, and enjoy MP10, which I can fairly easily - but much more slowly - do. The problem with both MP8 and MP10 is simply that the "bang for your buck", the XP gain per hour, kills per hour (which translates to more legendary drops), just favours - and by a large margin - the lower MP setting.
Since you asked for suggestions, I'll give two:
1) Either, lower the vitality - but not the damage boost - to higher MP levels (let's face it, if the mob hasn't killed you after 1 minute, it's not likely to after a second minute has passed), or;
2) Raise the bonus scaling so that the higher MP levels, especially 8 and above, have a much greater increase to XP, MF and GF.