They actually had an official response about the directx thing a few months ago.
http://www.rockpapershotgun.com/2011/08/03/why-steam-makes-you-install-directx-repeatedly/
Something to do with not just checking that you have the latest version but also ensuring that the specific and maybe older versions of libraries/binaries required by the game are installed. Still even knowing that, its annoying, probably there more to stop hail of 'WHY MY GAME NO WORK' threads from the minority than to actually help most of us, but one things for sure its not going away any time soon.