If your usage habits are such that you're not causing the OS to incur swap outs at 8gb, then you'll not need 16gb.
With the MacBook Airs, and the Retina's MBP's non upgradeable ram, it makes sense to go the 16gb route, even if you're not using it right now.
With the non retina MBP being upgradeable, then why spend extra money on a resource that you'll not fully leverage, especially since you can upgrade later.