RDM is a very bipolar community, it's split between those that want to be able to melee and others that just want to be backline mage jobs. You can't really blame SE for making Temper work on RDM only. The whole point of Temper is to allow them more of a melee opportunity, to allow one to cast it on other jobs would just defeat the purpose of Temper and turn it into another status enhancement for classes.

If you look at the big picture RDM is getting a nice balance of update
Melee=Temper
Nuke=Thunder IV
WHM=Raise II(ability to raise two people at a time rather then one is a nice utility to have)
Spontaneity:

This JA in itself has a huge varety of utility, if the casting is near instant to that of stun then it would be possible to raise 3 people, use a tier IV nuke instantly to finish off a mob, allow for back to back stuns if the timer is also reset. Odds are the ability duration is one minute so it would be possible to activate it to anticipate a time when stun is needed and you might need another one handy.

It just seems like people are missing the point as what Temper is. it's not meant as another enhancement to give to players but an optional utility to improve the melee power of a RDM if they wish. In the old era of the game the ability for a RDM to melee wasn't as great as it is now.

If you think of WoE, Dyna changes and even abyssea. The ability to melee and use magic is pretty important overall. In cases when it's not wise to cast magic off the bat the best bet could be either stand back and cure spam or help deal damage and create SCs, which adds another utility to the JA. a RDM could activate said JA open or close a SC and still have the ability to MB with a tier IV spell. With Temper by their side they have the chance to build TP even faster without having to use a sub or earring to give them double attack.

Overall you just need to look at the changes on a more grand scale rather then a small test size.