I've generally had a talent spec that gives me all the talents I really want for bear-form tanking, and most but not all of the talents I really want for cat-form DPS. With these talents, I do solid DPS, but I rarely top the damage meters.
Patch 3.1 a week ago brought dual specs, letting a character have two different talent specs. I bought a second spec, with the eventual plan of having one spec for the tank/DPS role I've been doing, and one spec for healing.
But we're running low on tanks in our guild these days, so it's not clear how soon I'll be needed as a healer. And I haven't enchanted and gemmed my healing gear yet. So when I joined a 25-man Naxxramas raid that had enough tanks, I decided to go with a full cat spec and see just how well I could do if I was focusing purely on DPS.
The answer: well enough to hit #1. Now, we were missing some of the folks who usually top our damage meters, so I'm not convinced I could hit #1 every week.
It was especially gratifying because the #2 player was a good friend, and I started out a bit slowly because I was still choosing talents as we were starting to clear trash. So he was cheering for me as I rose in the ranks, then as I was running neck-and-neck with him, we had the friendly trash talk of two guys being good sports.
However, having topped the damage meters once, I'm not eager to do so regularly. I'm one of our most solid tanks; I don't want to create bad blood by being our top damage dealer as well.