If you're waiting for the big multi-spec gear shift when patch 6.0 drops, you'll have to wait a little longer. Rygarius took to the forums today to clarify
that the gear that drops in SoO will not be changed to the system announced for Warlords of Draenor
Rygarius - 5.4 SoO Gear Conversion
Gear obtained from Siege of Orgrimmar won't be multi-spec. That's for the new items that will be obtained in Warlords of Draenor. There's no change to existing item levels and they'll will retain their upgrade ranks. If they were reforged they'll be returned into an unreforged state.
Some of the details are still being worked out. We'll be posting more information once there's more info for the remaining stuff.
Basically, this means that you'll still need multiple sets to do things like tank or heal or DPS as long as your wearing gear from Mists of Pandaria
- it's not until Warlords
itself comes out that you'll get the gear which switches when you do.