"pure" DPS classes have always had an advantage and most likely always will, just like hybrids always have a disadvantage due to the "lottery ticket" factor.
What this basically means, is that any spec will have a set job to do be it DPS/tank/heal, and depending on where that spec falls on the balancing lottery it will either be average, above average or below average at that job. This is where "pure" classes like mage/warlock/hunter have their advantage, because they get three tickets for the DPS balancing lottery, three chances to be above average whereas classes like Paladins and Priests only get one and some like Warriors/Druids get two.
This is essentially why DK's were so OP as a class in WotLK, because they got a total of six tickets.