You quaff potions as CW as well (that is, until you get the cleric companion), but you have the magnificent ability to teleport allowing you troll bosses and other players around forever.
Okay, not forever but combined with control abilities that slow down and outright stop enemies, you are pretty untouchable. If you took away the teleportation, CW would be pretty screwed class. Because while casting they can't move at all. And they are casting 90% of times.
In that example where I fought both GWF and cleric, if both had attacked me at the same time, I'd propbably wouldn't had killed either. But since cleric concentrated on healing, I managed to take down the GWF before dieing.