Well you are just thinking about roles in the sense of Tank/Support/DPS, but there are also subroles you could say, Melee/Range/Magic, so it's very possible that abilities could fit into two categories and each job will fall into two roles. Likea NIN would be DPS/Melee able to grab abilities from the DPS and melee roles while a healer is Support/Magic, BLM DPS/Magic.

So the Support role gives access to protect/raise while the magic side gives access to Swiftcast/Surecast. Though this does mean that healers could potentially lose access to Ice II if it's IDed as a DPS/Magic spell and not just magic.