Healing sages in PvP are also extremely weak compared to the other healers.
Smugglers/Agents are loaded up with instant casts, stealth, great AoE CC, good single target CC, and more burst than any of the healers
Troopers/bounty healers are the kings of single target heals and wear heavy armor. They have defensive cooldowns and are much harder to run interference on.
Sage/Sorc healers have no instant heals making them extremely vulnerable to interrupts in PvP. They do have bubble which is nice, but they are extremely squishie an as such extremely easy to kill compared to the other healers.
Really if you look at the facts, sorc/sages need some slight buffs for PvP.