I dunno about that. They're limited to one element, sure, but that one element can do a lot of disparate things, at least if you aren't just choosing Fire.
You can justify a whole lot with just one element, if you do it right. And I don't have a problem with that, it's a cool thing to do, but it's not how I think about the focused practitioner. For me, a focused practitioner is very good at the one thing he can do, but he can't really think out of the box as much as a full evocator could, because of his narrow field of expertise.
Fire is heat, heat is random movement in a gas. Spirit can do telekinesis, telekinesis is moving things with your mind. Conclusion: I can create fire with spirit.
Air is the element of motion and movement as well. I could move something so fast, that it catches on fire from the friction. If I do this on a miniature level, it will be indistinguishable from fire magic.
Those applications are well and good, but in my opinion, if you want to play someone who can do air and fire magic, just take evocation and have those examples above as your narration.
I would be more at ease with giving a focused practitioner (air) the wings power than letting him throw lightning. I see there being far more channeling options than there are elements for evocation, because some of the applications of an element are only fully developed as your knowledge about the workings grows. But that might just be me, I know my views on some of those things can be a bit weird.