There are several good reasons not to purchase a 13" rMBP today.
The introduction of Haswell (likely 3Q 2013 earliest for notebooks) will provide a significant performance upgrade for the 13" retina. The 15" retina not so much, as not relying solely on the Intel HD 4000 GPU. As is, the 13" retina is under powered and over priced, particularly in comparison the 15" retina models.
Either size retina model have experienced reported teething problems, such as the advent of image retention only many months after purchase. Yes, it is a quite lovely screen, but the price paid in part this uncertainty if opting for a Model A version.
The price, at least in relation to performance, will likely decrease with release of the next iteration. While lovely to have one of these retina MacBooks now (assuming developing no problems), this honeymoon may be quickly over before close of this year (in consideration of resale value, and of what one might have waited for).