I think it is not so easy to make a 3 DPS 1 healer 1 tank dungeon with how the roles are made now. If the difficulty is increased due to 1 more DPS present, the healer may have too much work, healing one more person. The strength of healers is made to heal a 4 player group, adding one more may be difficult to make without it being too easy (so that the healer can cope) or too difficult.