If you plan on fully exploring the game, you should probably do them. I know the one that gives you a nice Mace requires you to hit up several lower lvl instances. It's a great mace and will carry you well into the late 20s (early 30s?). But it comes down to effort. If your goal is to get to 70 as fast as possible, don't bother with them.
I did some, skipped most. It's simply easier to skip most. As of right now, the cost/effort to get my 60 mount seems prohibitive. I might just buy my racial mount and forget about it. (I'm 63 - that's pretty bad)...