So did you try out to install a game like Battlefield 1942 from release CD? Searching hours for patches and then another hour for a working mirror is not what I think is amusing. Same goes for the most mods.
Imagine you downloaded "that little code-mod" back in the day but you never played it again so it stays v0.1 and someday you just want to give it another try. now go, find the modpage, fiddle through hundreds of frontpage templates before you find out the current version info, find out what patch you need (b1, b2, b2a, b1.2, b3...) and then get a mirror which will not make you suffer the 1kB/s death.
If Steam would allow a SVN-Like distributor embedded in its gui and if they want to save Bandwith mix the server-side dist with an additional peer2peer exchanger even the smallest mod would stand a chance.