Fact 1 : WHM already has Reraise
Fact 2 : The only time the WHM does not have Reraise up is when he forgets it, dispelled by some effect (capped areas, BCNM fights, or Dynamis COP zones restoring subjobs), lost the effect barely a few seconds before dying, or didn't have the MP after Raising back up.

Giving an Auto-Reraise (Raise 1 effect) trait would only serve as a failproof against these possible instances. EXP loss is arguably a concern of the past with Abyssea, you get it back in a few minutes worth.

On the other hand, more and more jobs do have access to Raise now, as subjob levels have caught up allowing even RDM subjob to have a Raise handy. Unless you are the sole healing type in your run, there shouldn't be a need for such a Trait. If you are too busy fighting a big monster to think about Reraise, it's probably due to bad planning as you should have casted it before starting the boss fight.

Basically, while a good idea, I'd like to avoid tossing the blame at SquareEnix for people's shortcomings when they happen :P They don't have to add things so you don't have to do your job.