I haven't tried Diseaseless on bosses yet, simply because putting up diseases is habit after more than a year doing so. I've heard several players talk about how much easier it is, and I can certainly see using a DS instead of refreshing diseases situationally if you need the heal/shield. Here's Ghostcrawler's comment about it:
In the case of "diseaseless Blood," we don't have a problem if tanks choose to sacrifice some of their threat generation for a simpler rotation or other benefits. It doesn't bother us unless ignoring diseases becomes the only reasonable way to play (and with Outbreak available in Cataclysm, applying diseases is even easier). When dps specs were ignoring diseases, we made changes, because the whole DK rotation was based on applying diseases. That's true to a much lesser extent for tanks, but we try to have pretty simple rotation for tanks anyway because they have a lot of other things to worry about and are often having to deal with very dynamic situations compared to what a PvE dps DK might be doing.To me, he's stating that they don't really care how people play now, and that if a few tanks don't want to use diseases it's no big deal. However, if everyone stops using them, then they'll make changes; if you read between the lines, he's saying that going Diseaseless is intended to be sub-optimal. If it ever becomes the ideal rotation, then they'll either nerf it or buff diseases. After reading that, I don't see any point to dropping diseases from my rotation as it's clear that we are intended to use them. As GC says, with Outbreak it will only take 1 GCD to apply them, then 1 more to spread them. With Epidemic they actually last quite long, although I won't have that in my spec until Cata hits.