I wonder many games do people play at a time where they always need access to it? I'm probably an outer edge case since I only play one game at a time, finish it, then move on to the next game without going back to the last game I just finished, since I mainly play single player campaign games, so replay factor isn't huge for me. Therefore, for me, I would never need 20 games installed at once anyways as I wouldn't be going back to play most of those games I already beat again.
However, if I have the space I would just leave games installed even if I beat them already like on my comp where my steam folder is 700 gigs right now, but space isn't an issue on my comp.
I usually do play straight through a single player game when I game after getting it, but I also do go back frequently to replay games if I enjoy the experience or there's more to see (e.g., Paragon/Renegade in ME, save/harvest in Bioshock, etc.) And then my 8 year old often plays quite a few different games, at least on 360 - he'd fire up Blue Dragon, or any one of about eleventeen trillion LEGO games depending on what he feels like playing on a Saturday.