For me, one turn off is that it just has too much features, too much going on. So where it has advantages, it's nothing i would use over an mpc, i'd do things elsewhere, like my PC. To me, after seeing someone using it, it almost seemed like software was the alternative to it, not the mpc.
One thing that i like about mpc's is that soooooo many people buy them because they think it's going to be for them. I know countless people who bought an mpc that didn't sample, just because they thought that it would be something they would use. I know two people who bought a 2500 and just didn't get that unless you have other synths, it's just an expensive drum machine if you don't sample. So you can get a basically new mpc for nothing. I'm not seeing the same prices on MV's or it would be a concieration.
The obvious is the reputation. The other thing is that peice is HUGE. Space is a big deal for alot of people as well.