Depends on how long you keep the machine. Those talking about future proofing know 1st hand, that the RAM as well as GPU are the 1st to show signs of age.
My wife inherited my 1st gen unibody C2D MBP. The machine is plenty old now, but all it took was some more RAM and she is much happier now when she #$&^@$% refuses to ever shut down programs she opens and is no longer using. The machine went from unusable to her to very usable with the simple addition of some RAM. Sure, you would say that it's because she is not shutting down programs. But in reality, she is simulating a machine under load.
With the way Apple is progressing, the kernal task will be taking up AT LEAST 2gigs of RAM in another year or two. Also, all it takes is for the OP to start experimenting with any program that eats RAM, and those 8Gigs are toast.
You laugh at future proofing, I say get more now, since you can't upgrade. And you won't have to live in fear of actually doing something new with your rMBP.
Then again, if you feel compelled to get the latest and greatest every calendar year, the concept probably is a little foreign to you.