You can already [give extra copies][1] of games you own and get more from a pack (*in [select circumstances][2]*) on Steam.

I can't imagine why this would ever be an idea considered by Valve, however.  The prices of games tend to go down over time anyways, so if you want it for less money, you could just wait.  Publishers who use Steam would need to agree to this somehow as well, which seems pointless if they can just sell a whole other copy of the game for the cost of a few GB of bandwidth.  

Sure, *users* can clamor all they want for it, that doesn't mean it will ever happen.


  [1]: https://gaming.stackexchange.com/questions/20182/what-happens-if-i-buy-a-game-i-already-own-through-steam-a-second-time/20183#20183
  [2]: https://support.steampowered.com/kb_article.php?ref=4502-TPJL-2656