It is a 30% (soon to be 15%) heal AS WELL AS RESETTING BLOCK; yes you have to wait 30s to use the second one but if you're taking that much damage in 30s you don't deserve to live.So far out of the fights I've done, G Invis is only good on H Elegon. I've heard it's good on other fights too, I just haven't gotten to them yet, and if you really want a 30% (15%) heal over a safety net like Cauterize, be my guest.
Now, it may well be behind GInvis and Cauterize; this doesn't make it 'useless', and there are a few (a very few) situations where Cold Snap will save you in PvE encounters this tier where Cauterize and GInvis won't.
Yes, Mages have been benched this tier at the top level of raiding, but so have almost all classes; getting a raid spot as a Boomkin during progression this tier was near-impossible for those in top guilds just because other classes do what they do but so much better.
Honestly though, looking at HCs, I count 4 fights which require 'a lot' of movement. If there were more honestly Arcane wouldn't be as viable as it is because mobility does kill it. Sure it's possible to maintain 6 stacks while on the move and spamming Scorch, but that doesn't mean it's good DPS.
Homogenization is the worst thing about this game; people cry about other classes having X while their class only has Y and so Blizzard fixes it to keep people happy. If it carries on there's no point in even having 11 classes; might as well just have 2 classes; Melee class that can tank/dps and Caster class that can heal/dps.
I know you don't want homogenization but adding more mobility to Mages will only make them more similar to other classes and that is exactly what homogenization is - everything becoming similar/the same.