Having a hard mode available for individual bosses damages the flow of a raid. Traditional raiding had the "rising tension" timeline of a good movie. You started out with some easy bosses to get you into the rhythm of things. Then you'd get to the ones you'd gotten down but still might need a few attempts on. Then . . . it was time for chewing glass. Maybe you'd get that new boss down tonight, maybe you wouldn't. But you'd keep trying until you were exhausted.
Ulduar's changed that. Now you can start chewing glass with the first boss of the night if you leave enough towers up. Sure, after a few wipes you can back down, take out a tower or two, and get past him, but you've still got the stress that comes from repeated wiping. That's roughest on any new members or substitutes you have in the raid. Instead of getting a chance to show they can pull their weight with the rest of the team in a routine fight they get stress-tested and probably wind up as the "weakest link." Let's face it, unless you recruit the guy kicked from Uber-Guild for only raiding five nights a week instead of six the new folks are going to need a bit of practice to catch up with the regulars. Even if you get the guy from U-G he'll need to learn the pace this group goes at and how they handle the encounters (Oh, new guy dropped the bomb in the wrong place? That's probably where his old raid told him to drop it). That's not a big deal if you're farming an encounter. If it's a notch up from the previous week's run the new guy's probably going to cause a wipe, or just place last in dps while learning where to stand.
From new guy's point of view he's probably switched from "Yay, they let me come tonight!" to "OMG, are we going to make any progress at all or just wipe all night?" Not a great way to recruit new regulars.
For the regulars early glass-chewing isn't good either. You go in with only so much mental energy. If it's all used up on the early hard modes you'll be exhausted when you get to new content at the end of the evening. Not the frame of mind you want for tackling something new and difficult.
Fortunately Blizzard seems to be looking at boss-specific hard modes as a failed experiment. The 3.2 raid will have "normal mode" and "heroic mode" as separate lockouts. So this problem will be limited to Ulduar. In the mean time let's save hard modes for when we've cleared the whole dungeon.