Mages are pure DPS, period. How exactly does one heal with Time Magic? Is it about avoiding damage instead of healing it? They could probably make a more support-oriented Mage spec, but never pure healing. It simply wouldn't feel right.
Warlocks would work as a tank, the glyph indicates that, but it won't happen. The fact that Blizzard nerfed the crap out of DA before it went live to crush any hope of it being a viable raid tank says a lot about that. I'm okay with that, though. Warlocks as a tank don't exactly feel very...right. The problem also arises in trying to set up new stat distribution. Stamina stacking is boring. I have high hopes for a tank/dps Demon Hunter class, but don't touch my warlock! As for healing...not going to touch that. Warlocks healing makes me cringe.
Rogues have filled a very small, almost non-existent tanking niche for specific encounters, such as Garalon, but a full blown tanking tree would be silly. Or maybe not? Ninja tanks in FFXI were pretty nifty.
TLDR: doesn't make sense, we need pures.