Certain bosses will not allow for you to be tanking as a warlock, because <insert other tanking class at random> is many times better for the given fight. Much like warriors vs DKs for Ra-den.
If you're ready to be benched for all hard fights, then you already have a perfectly viable tanking spec. What's the problem? I bet you'd have alot of fun sitting on the bench whining about how under-powered your class/spec is. Especially after having gone through hell playing that character since vanilla, just to have it rerolled to a COMPLETELY DIFFERENT CLASS after 8 years.