There was a time when 20-man raids were considered the fun/casual thing to do in comparison to 40-man raids. Now even that's too much to ask, apparently.
Fact is, by forcing people to pick either 25 or 10-man, Blizzard is messing up a basic mechanic that has been there since early Vanilla. There has never been a scenario where, in a large raid (25-man) you could just give up, drop down to a smaller raid size, and get the same loot. It seems pretty obvious to me that this will kill larger raid sizes any time there is any actual difficulty.
If the content is just on farm, then sure, people may do 25-mans for faster loot. But otherwise, it's always going to drop down to the small group sizes. It's just common sense.
Furthermore, anyone who raided both 10 and 25-man raids will be screwed. Want to play with your friends outside raid time? No dice, roll an alt or something. Want to progress independently, and still be part of a large guild? Screw you, the casuals don't like your playstyle.
IMO everyone who thinks this is a good change is just a spoiled brat who can't stand that there are things out there that other people enjoy to do, and that they just might be rewarded for doing it. You can't use the argument "oh I don't play for loot" and at the same time say "waaah 25-man raiders get better loot!" That's just a crappy argument. And there's really no defense for saying "25-man raiders shouldn't get to raid both, because... [insert bad logic here]."