I am interest how the cMP can replace your MBA. In general, people buying MBA for it's form factor, probability, slim looking, etc. None of these exist in the cMP.
Anyway, may be you just want to emphasis that you know how to use OSX / Mac.
For me, it highly depends on what you want to do.
e.g. For Video editing in FCPX, yes, the cMP still one of the best machine you can use (with proper upgrade).
However, for gaming, it's much better to build a gaming PC.
For media server, the cMP can do the job well, however, with extremely low power efficiency (compare to other media server options).
So, why you want the Mac Pro? Without knowing that, it's hard to tell you if it's worth. Or which config may be the sweet spot for you.