There are clear reasons why many players don't like these mechanics. Who wants to go to all the trouble of collecting a whole set of gear that you use only for one fight? Who wants to let that gear take up all that bag and bank space? Who wants to be shut out of their favorite talent build or sit somewhat on the sidelines just because the bosses they're fighting are immune to their prefered style of damage?
So Ghostcrawler comes in with the developer's logic on this issue: They don't want the raid instance to be a simple thing where you just move from one boss to the next boss, to the next, and so on. They want to break it up a bit so that different bosses require not only different strategies, but different gear, and different abilities, too. This adds a bit of anticipation, of having to get ready for the challenge rather than just stumbling into it and accidentally getting it right on the third or fourth try.
Personally, I can see how this makes sense from a developer's point of view, but from a player's point of view I do think it gets old and uninteresting, especially if the same mechanic has been used a lot in previous resist and immunity-limited boss battles. It might have been interesting the first time we saw fire resist battles in Molten Core, but even then it quickly got old -- now in Northrend it would hardly be new and exciting to see a bunch of battles requiring frost resistance. It's just more of the same old thing.
Ghostcrawler admits, too, that Blizzard hasn't always got it right, and she hopes that helps explain why resist and immunity battles were put there in the first place. Regarding how things might be in the future, she says they are actively discussing these sorts of resist battles, and they are wondering if "it may be that the time has come to phase them out." No promises or anything, but it's definitely a question on the table.
What's your opinion? Off with resistance' head? Long live resistance?