The reason Steam continues to separate itself from the rest of the online distribution world is how friendly they are to the way YOU want to play your games. I have been a happier gamer since building my Steam library.
I can uninstall any games I'm not currently playing.
I can reinstall them four years later (and I have, all the time) to play it again for a while.
I can uninstall it again.
They don't care! I love them for this. They hold the games for my use longer than I can keep track of the physical media. And having them never complain about bandwidth as I install/uninstall or put limits on it... is why I keep buying. I hope they never EVER change this. It's just the way online distribution should be. Thank you Valve/Steam.
Don't forget, you can go to your buddy's house. Log in with your steam account and play your games on his computer!