What would be the reason to purchase a non-retina mini 2 at this point?
Maybe the usual slate of: faster processor, more memory, more storage, longer battery life.
Remember, the mini 1 is functionally an iPad 2, which is at the bottom of tolerable capability/capacity and is the next tablet slated for obsolescence.
As for non-retina, pixels on the mini are smaller than the iPad 1&2 so it's heading toward retina territory anyway. To go full retina,
has to shrink the already crammed iPad 3&4 pixel density into a smaller space (or else, in a very ugly move, create yet another "standard" screen dimensions), pushing far into retina density without any discernible benefit beyond software compatibility. Someday it will be the norm, but for the next few months at least there isn't much benefit for
to make that leap; the cost/benefit ratio just doesn't compel the move.
The only reason I can note for a retina iPad Mini at this point is competition from others not so constrained to particular screen dimensions, selling on specs & price (and cutting corners to get there) alone without the walled garden imposing certain sensible limits.