PuG Desertion

Very occasionally, I post on the WoW Community on Live Journal - nto very often, as they're not the friendliest of folk, but I thought I'd share something I posted there just recently, and get your opinion. I'm not sure how many of you play WoW, so it may not be appropriate, but you can always take a stab anyway.

Is it ever acceptable to leave a PUG before it's fully-formed, in favour of joining another which is only missing the class you play?

For example - I'm currently playing a 70 Protection Warrior, and I'd been sitting in a PUG yesterday for 10-15 minutes waiting for a healer, when I noticed another group for the same instance that was looking for a tank, and was ready to go. I gave my apologies to the group I was in, and volunteered for the other group. It turned out that the healer, a Priest, in the new group was in the same Guild as one of the members of the previous group, and had mentioned in their Guild chat channel that he was less than happy.

Although I had apologised, obviously I'd upset at least one player, so I was wondering, what the prevailing opinion about this kind of thing was?