Doing them in LFR? It's hard enough getting 4 other people to agree to do an achievment in a dungeon so good luck getting 24 others to agree in LFR.
I'm glad you can now get achievements in flex because it fixes a couple of problems with the current system. If you are a heroic raiding guild you have to stop doing heroic modes so you can do achievements on normal mode (yes I know a small ammount of them can realistically be earned on heroic). There's also the problem that a raiding guild can't fit all it's raiders into the raid instance at the same time. This means some players miss out on the achievements OR you have to use more than 1 raid reset to get them for everyone and then you are missing out on even more heroic boss kills.
Being able to do raid achievements in flex mode solves both problems.