Why is party member switching so bad though? It still requires teamwork like any other tactic, and requires people to be there and available. Each team member still has to contribute. I guess I just don't see it as an exploit but instead as a tactic. Difference of perspective.
If you have 18 people in the alliance, all damage dealers, and your healers are outside of the alliance, you could bring 30 healers to support your 18 main characters, and also bring a couple of extra hands, that would be swapped assuming one of the 18 member dies. Certainly, a form of strategy, but also certainly not the way the designers expected the fight to be dealt with.
If the 18 member limit suddenly becomes "18 tanks and attackers" with an infinite amount of support, then the fight either have to be balanced around that capacity or you keep making content meant for 18, that people bring 30+ people to do, and trivialize it.
It's a slippery slope -- if you keep designing assuming the players will play "fair", then the players who use those "meta-tactics" can easily destroy your content, and ask for more. The typical designer response is to make harder fights, to keep the achievers happy; something to strive for, a more challenging fight and new goodies to get. Rapidly, however, that content becomes nearly impossible to win without the use of said tactics, which means your less hardcore fanbase can't hope to beat it, and then you have to figure out who you're making content for; that 1% of super hardcore bored without content, or the masses that are just playing the game?
It's much more "clean" to prevent any of these tactics, design the boss fight for a fixed number of people, who cannot receive outside healing, and balance the fights around those strict guidelines. BCNMs, instanced battles, WoW raids and dungeons and such do not suffer from party member swapping, because you can only bring in so many people.
Also, being the benchwarmer who only joins the fight when the "awesome" damage dealers eat the dirt, only to drop party and let them back in 5 minutes later once weakness wears off is no fun. If you're present at a boss battle, wouldn't you rather be allowed to go and fight it, instead of helping out for 5 minutes every once in a while?