The path of least resistance. People will always find the easiest route to farm. Currently the rewards for a higher MP level do not outweigh the cost. Until they do, people will continue to take the path of least resistance.
Sure, its fun to challenge your charecter by raising the MP level, but most people only do that because they HAVE to. i.e. Key farming on anything less than MP3 is a waste of time. Because in MP0 the exp to HP ratio is 1:1 whereas at MP1 and higher the ratio becomes skewed in favour of HP, mobs take longer to kill but yield less exp proportionately. That means grinding for EXP is still the best at mp0.
The other rewards, such as higher MF and GF - don't seem to compensate well for the increased difficulty. It takes me far less effort to collect many, many more rare items in MP0 compared to MP5. I think i still find more legendaries playing mp0 then mp5.
Upping the chances for a legendary to drop in higher mp values would certianly offer some allure. (increase the MF bonus you get from higher mps to scale better with the health bonus they gain.)
Ultimately, it should go like this => Higher MP = better rewards (when i say rewards i mean exp and/or loot) . Weak or new players would want to work their way up the MP system, starting at mp0 until they were able to efficiently farm mp 1, then 2, then 3... and so on... though its not REQUIRED, better geared charecters should be compelled to play on higher MP levels. Farming mp5 should be more rewarding -in terms of exp gained and/or loot found- than mp4, or 3, and especially mp0... this would compell people to play as high of an MP as they can.
Though it shouldnt be so overly rewarding as to make MP0 farming obsolete (because some people really enjoy just running around 1 hitting everything and completely smashing mobs... which is fine), it should be just a little bit more rewarding each MP level you set it at...
One way, your EXP would scale proportionately to the monsters HP, so really as far as exp grinding there would be no clear and obvious "sweet spot" and any MP you do it on would yield roughly the same results (minus travel time and other factors)
So the allure then becomes that chance to drop additional treasure. Which, i think when paired with higher mf and an equal increase in exp, would be appealing enough to make players want to play on as high of an MP as possible.
Alternatively, one could increase the EXP gained throughout the MP levels (though that might give an unfair advantage for Paragon leveling, to the more powerful players) So that MP10 would be the best to farm for exp, with mp9 and 8 slightly worse, with mp0 being the worst.
All things being equal, most people will choose the easier route, so you have to offer some better rewards to get people to play on more challenging MP levels (if thats what you want), otherwise everyone will continue to take the easiest path to sucess.
Edited by Riot#1422 on 11/16/2012 1:58 PM PST