Well with the Glyph of Mend Pet and Thunderstomp change, it looks like Blizz might be starting to take the notion of pet tanking a little more seriously, so it might turn out eventually (not in this patch, however) that we may have a choice between tank/DPS roles, at least for 5-man groups (I doubt that our pets will ever be raid tanks.)
But yeah, unless your class has a spec that makes you a specific tank or healer, dual spec is a little wasted. "Hmmm, what should I be for this raid? DPS or, er, em...DPS?"
Edited, Mar 31st 2009 7:41am by Ambrya