A lot of people here, specially the Master Loot crowd, forget that in WoW the "middle ground" is way way wider then you think.
Of course a stable guild with mature member with 100% attendence, with 100% chance that the guild will raid till the new expansion can gain great benefit of ML, because eventually everyone will have the same loot and you will clear raid a lot faster. In this scenario, Master Loot seems to be the only reasonable choice.
But, but, how many guilds can be identified with the above?
The vast majority of Mythic Guilds are in the middle ground, and that means one or more of the following things happen: they guild members does not share goals, you have selfish people (also hidden); those people will sooner or later start drama; you don't have 100% full roster every raid, leading to the necessity of bringing outside players, with every consequnces in startegy and loot terms; the guild is very prone to disban, because you have half player very good that "carry" half people.
I cuold go on, but the point is clear: a full mature and stable Mythic Guild is not the rule, but the exception.
Blizzard doesn't care about the mature X guild and that X cuold clear Mythic in 3 less week if he was able to give gear with ML to the player who needed more. They care about the majority of players that can see Personal Loot as a incentive of stepping into a selective raiding scenario. THEN, after the player is matured, he cuold join a mature Guild where, even with Personal Loot, people are willing to trade gear for the greater good.
And yeah, i will be honest, this change will also make guilds take more time to clear stuff, and more time = more sub = more money (honestly i don't see this as a bad thing since they are not a Charity Society).