Or perhaps it is Apple's not so subtle way of encouraging you to use the lower price download option and weaning users away from receiving software on physical media.
I'd guess no, because the thumb drives were specifically targeted at users who wanted to upgrade to Lion but didn't have high-speed d/l capability. (I guess Apple's not marketing Mountain Lion thumb drives signals the advent of universal high-speed Internet.)
They weren't even marketed as a solution for owners of new Macs with Lion who don't have the high-speed d/l capability necessary for the only kind of re-install their Macs are capable of. (That, of course, would have recognized an ongoing necessity and mandated thumb drives for each new version of OS X, and, to change the subject slightly, the lack of such recognition may, no, probably will, ultimately spawn a booming business in bootleg OS X discs and thumb drives as Apple releases new versions of OS X, makes earlier ones unavailable by legal means, and totally screws late adopters in the process.)
Weaning us away from physical media? I'd say that Apple's more like beating us into submission.