MF has almost no effect on legendary items but it doesn't hurt your chances either, based on the way the tables work now. Different then D2 with diminishing returns and such. Since we are running specifically to look for a legendary item, and don't care at all about the others, then stacking valor for example is a waste, unless it happens naturally on your run. Same with getting extra MF items just for the sake of the run. Now if the cap was removed and we could reach say 600-800-1000% MF then it would have a slightly noticeable effect, but still not a enough to force the issue. .002% is just way to low to be substantially effected one way or the other because of how MF is calculated and the Pseudo-randomness of the drop itself.
I think you're undervaluing MF.
Not only does it increase, say, your chances of a Legendary roll from 0.002 or whatever at 0 MF to 0.008 with 300% MF and to 0.0095 with 375% MF (capped with 5 NV), but it also gives you an extra item drop from all elite/champ packs, which is another item that has a chance to be Legendary
The first point is actually far greater than you're making it out to be. The base chance for a legendary is not statistically insignificant. People have found Legendaries. People have found many Legendaries. All things considered, you should have averaged at least 1 or 2 Legendaries by now, and the people complaining about having never seen a single Legendary are outliers. Doubling, Tripling, or Quadrupling your odds of a statistically significant chance (0.002% is 1 in 50,000, which is pretty damn good considering how many mobs we can kill in just a couple hours farming) is great. It's not like we're buying two, three, or four lottery tickets compared to one ticket in a lottery that has like 1 in 1,000,000,000 odds of winning, to which I'd say yeah, you're not relevantly improving your odds. But when you're going from 1:50,000 to 1:10,562, that's a different story.
I'm not sure how powerful the 2nd point is, because on its own it might not seem like much (it probably isn't). But I'm sure something is up with lower level mob drop chances / mf / etc, because I see far more often than I should that the only rare I get is the 1 guaranteed NV rare.