My guess is that they don't want the MFer to be a leecher. When you stack MF you're most likely compromising your damage. I don't really like the way it is either, but I wouldn't want to carry the weight while the MFer is getting all the loot.
Rares can have main dmg stat/vit and MF/GF and still room for other useful prefix/suffixes. There is only a compromise if you're hoping for other goodies (life regen, hatred regen, etc.).
I really wish Blizzard would change their stance on it, b/c I would much rather MF alone than having some douche with 0% MF killing my drops.
We already get our own loot. We should get our own MF/GF. Period.