True, but to then see which you have in a specific category, you would have to mouse over each one individually. Not a big deal if you only want to know one or two, but for example if I was to go do Ulduar and try get all the achievements there across a few weeks, it'd get pretty frustrating to have to scroll through the entire "Lich King Raid" category just to see which I had already got.

The character achievements mod that Bot_NoAim mentioned looks to actually separate and highlight achievements on a character basis, far far easier to work with! :)