rMBP makes more sense since we are accustomed to seeing a lower case "i" in front of various Apple products among other things so the lower case r looks cool. Besides I see "rMBP" more than "MBPr" so no. I don't like saying MacBook Pro with Retina because its too long and anyways it sounds like the same product with a retina screen when its really not. Just my opinion though.
Anyways I'd like to create my own abbreviations for products and have everyone follow them but thats pretty unrealistic since it "just happens" and it sticks.