With WOTLK, you will be seriously hurting if you don't quest to open up certain factions / areas.

Much of the content in Storm peaks and Icecrown is completely phased and requires doing a lot of the available quests just to open them up.

I would also tend to agree that the questing exp and the money for 5 toons is probably going to be better than any instance you care to run.

Yes, there are some annoying collection quests, but there are also a lot of "do it on one, and all get credit" quests so that in the end, the money and exp make it worth it to grind through the collections.

Just my $0.02