If you know the effects of MF on gear quality then I'm sure people who can farm MP8-10 will definitely do it. Insanely high chance of getting good items, even if it's slower. Paired with insane bonus XP and gold/run. I would do it if I had the gear for the challenge and the guaranteed reward.
Going from MP 8 to MP 10 more than doubles mob HP. Now even at MP 8, I think the timing of runs is mostly dominated by kill speed, and much less by run speed. So I wouldn't be surprised if the same run takes twice the time on MP 10 compared to MP 8. Now what do you get for that? 50 MF, that's probably a 10% increase since one sits at (200 + 75 + 3*plvl) ~ 300-500 mf anyways. And slightly higher item drop rate. That's not enough to compensate the slower kill speed.
If you can kill the Ubers with MP 10 that's nice, and probably worth the effort since you get guaranteed drops. But for legendary farming, I doubt that it is the most efficient setting even for super highly geared (200k-300k) dps players.
The bottom line is that nobody needs to be concerned if s/he can't farm high MP levels. Even at low MP you get more MF than previously, and now that all rares can be good and legendaries drop more often, it's much more likely to find a good item yourself. So everyone wins.