So it's really that big of an 'annoyance' to see the same content for a few weeks while working on achievements and then heroics.

Besides, if it's really that big of an 'annoyance' (a word I keep seeing pop up here), then don't worry about the achievements. Kill the bosses on normal, then kill them on Heroic. Or wait, is killing the boss twice too much of an annoyance? If so, why play. I don't mind working on an encounter several times. There are opportunities for personal challenges each time you go in, and each time you go in, the fight changes if only very slightly. I guess it helps that I'm in a guild that allows different people with different classes and levels of abilities into the fight - that allows for further diversity in how the fights are tackled. For the hardcore guilds that have a set number of raiders that never changes, and focus on only one way to down a boss, then I suppose around 10 boss kills it would start being monotonous. But I still think it's good to allow other people to down content they wouldn't be able to normally because Blizzard nerfed the original encounter and added challenged for the l33ts.