First of all, it isn't my goal to criticize perfectraid with every post
I used perfectraid before wow 2.0 and am waiting to tell my guildmates about it when it's ready to go. Thanks Cladhaire for the rewrite and new features in progress.
On to buffing:
I don't know how popular/useful/difficult this would be but in my experience you're usually in charge of buffing your own group, which you have support for. You're typically not in charge of buffing other groups with your class. Groups that don't have a representative of your class are up for grabs, and I think most of the people reading this thread are the ones who buff those groups in the end
I have 3 points:
1. Love the "My Group" idea.
2. It would be great to have an option for "groups without my class in it."
3. For priests, druids, and paladins (not familiar with shaman at all) there are talents that enhance buffs or actually grant the buff. An exclude list would be great for the above "groups without my class in it" selector. For example, your guild shadow priest might have imp fort (follow me here). So, he wouldn't be in fort's exclude list, but would be in spirit's exclude list. Or a druid without improved mark of the wild... let's say you keep getting tells from people in his group that his buff isn't the improved one. Well, add him to the exclude list for MotW and if you're buffing "my group" and "groups without my class in it," you will buff the druid without improved MotW's group. When the raid leader switches groups around, you don't have to do anything.
This option might seem like a lot of work on the user's part, but you know your guild and the people you run with. In TBC, for most folks this will be even less people.