Would it be nice to get, sure.

Does it fill any purpose we are currently lacking in? Not really.

You have to keep in mind that the devs have a limited amount of time and effort they have to split between everything so while ultimately it would be nice if we were to receive every spell and ability on the wishlist topic that just isn't feasible. Given that they have limited time and resources, changes that don't really serve a purpose besides being a situational new toy are better off yielding to changes that significantly add to the job. This is partly why Shield Mastery pisses me off. Sure, getting something is better then getting nothing (unless that something subtracts :P), but compared to all the potential changes they could of made that would meaningfully change the job for the better it just felt more like a waste of time (at least in it's current form).

So yeah, would it be nice to get Runic? Sure. Would it be nice to get Double Cast? Absolutely.

But compared to the thousands of other changes that they could make to the job that have significant meaning especially when some of those changes could very well fix current problems with the job I can't say that adding Runic would be a very efficient use of their time.