If by future-proofing against OS X updates you simply mean that if OS X in the future ends up running slower on older machines because of higher realistic base hardware requirements, then okay, fine...maybe. But if you mean you are hoping to protect against your computer being completely unsupported by a future version of OS X at some distant date in the future years down the road, you should know that Apple has never, to my knowledge, discontinued official support for only a subset of a generation of a particular model of Mac in any given OS X release. If a future OS X 4-5 years from now ends up not supporting 1st-gen rMBs, it will fail to support ALL models, not selectively fail to support the lower-clocked ones while continuing to officially support the higher-clocked ones. So holding onto a 1.3GHz model will give you no advantage in that regard.The reason I keep thinking, though, that I should KEEP the 1.3 I preordered is "future proof" (i know there is no such thing, but more like "future better"). OS X updates and the like. Keeping up with new code for Safari. Etc etc etc.
-- Nathan