It makes the defenders actually earn their keep.
Yep.
As a DM running intelligent baddies, I want to go after the Strikers first, or anything dealing large damage. This is not only the smart/realistic thing to do, but is the reason why a role exists and gives them a chance to do their thing. It's ASSUMED by the designers the baddies will go after certain types. Otherwise, why not play a group of all strikers with a controller?