I'm not talking specifically about Twintania, furthest I've been so far is turn 4, but never a good idea to dps a bit in coil? I can think of a few places, particularly the aoe phases in turn 4, most group will at least have the white mages help out a bit with holy in the aoe phases. I'm really having trouble finding the consistency in your argument though, if you don't have the time to dps (which is perfectly reasonable and expected in much of coil) you should be happy to get a 3% boost in throughput. I would never advocate a healer dpsing instead of healing, I advocate dpsing instead of wasting time overhealing or doing nothing while waiting to need to heal.

An upgrade is never useless, if you already meet the base requirements for your job without it, then it will give you more margin for error, or allow you to exceed your base role.