Is defender vital? No. It is probably the most helpful class in the game for pve content and makes encounters drastically easier. Striker and sunbreaker are obsolete in end game pve content now. In the same way that Nightstalkers are not essential in the raid. But they do help an awful lot. You will very occasionally see a gunslinger in the raid, but never a blade dancer. When you see lfg posts saying 'need two Titans' they aren't looking for strikers.