IMO, if you are playing a defender, having sky high defenses is pointless unless you also focus on making your mark mean something. The ideal is defenses that are tempting but still high and a really nasty punishing attack for anyone who ignores you.
Actually, this is somewhat misleading: The ideal situation is to have both a crippling punishment mechanic, and to be nigh invulnerable. That way any enemy that you mark loses either way. My hybrid paladin|fighter has lay one hands (house rule), high defenses except for his reflex, resistances, and THP out the wazo. If you choose not to waste time trying to hurt him though, enemies eat 9-12 automatic damage, a melee basic attack, and lose half the damage from their attack, and he can do this twice a round. Both options are poor ones for team monster, particularly if team monster is a solo. You shouldn't reward team monster for trying to fight a defender, you should make that suck and any OTHER option suck even harder.
Its the difference between a good defender and a great one.