No one in the current economic climate has startup funds to double up on every machine! In fact the rMBP is one we got as a replacement for the previous ones that got stolen. The portability being the main reason for the upgrade as i'm constantly out in the field or different offices etc...
For the cost of one base model rMBP one can purchase two refurbished 13" MBPs, with money left-over to boost the RAM in both Macs (in the U.S. anyway; I'm not familiar with the U.K.)
That is how I would budget my limited portable Mac purchases until I had the funds to upgrade. I've had to settle for something less that I would have preferred due to money considerations. Since portable computers are prone to theft and damage, redundancy would be very important to me if I depend on them in my business. My work involves using valuable and easily damaged gear on location; having adequate backups of critical pieces of equipment are not luxuries, they are a basic cost of doing business.