On the other hand, it can provide a nice alternative if you don't want to play a warrior, but also don't necessarily want to drag one of your warrior companions everywhere to tank for you.
My Lavellan is a mage and her favorite companions to take places are the rogues and fellow mages (sans Vivienne). I grew to really resent having to take Cassandra or Blackwall everywhere, and thus give them a companion slot I could have given to companions whose party banter/comments I really liked.
As for Rift Mage vs Knight-Enchanter, I couldn't make up my mind in the first playthrough, so picked no specialization. They both seemed to make sense story-wise. Lavellan could want to learn more about the glowing green mark, or want to learn a skill with elven roots (then learn it is indeed like the ancient Arcane Warriors). I didn't really like the idea of both Solas and Lavellan being Rift Mages for playstyle reasons though, because I liked the idea of each party member contributes in their own way. Since I need to tank but don't want to be a warrior, KE works.
Yeah, I like rolling with my warrior companions, but I have a lot more fun with a melee mage than simply playing vanilla mage or vanilla warrior. While everyone else was complaining about the horrors of a couple companions being bi, *this* was one of the things I missed in DA2. The melee staff was a good touch, but it wasn't the same. I like to alternate between long distance, and then zooming in there and going melee, depending on the situation and what I feel like. Do I feel like static cage and nuking them into oblivion without moving from this spot? Or do I static cage them, fade step swoop chill them, then destroy them with spirit blade? Or maybe I'll skip out on the static cage entirely and just have at them with my spells of choice with the occasional spirit blade when they get too close. That annoying demon trying to pop up under me again? Phase out and laugh as they land on my detonation--whoops. It's the best of both worlds. I still get to play mage, but I don't have to stay at a distance from the action.
The lore attached to KE is an added plus for an elven PC too.
Heh. I actually deviated a lot from the 'standard party setup'. Rolled with 4 mages for a long time, and we utterly destroyed everything. Don't really need a tank when you've got 4 rotating barriers.
And you don't need a rogue when you can do stuff like this:

How did...? Is that gif from a mod or something?
Really, I've wanted to do that so many times.
And yeah, four mage party is awesome until you hit a locked door. 