I also rarely play more than 3-4 games simultaneously, but with the push toward digital downloads, I foresee this becoming more of an issue. I personally don't have a bandwidth cap with my ISP, but many people do. Eating up 50 GB at a time, it won't take long to reach that cap. I would prefer to download and install the game at my option, then, play whenever I feel the itch.
Yep, a stock 500GB HDD has something like 465GB after partitioning (they measure HDDs in an arguably deceptive way for marketing speak). Bet on a modest hidden partition for recovery, say 25GB, takes it down to 440GB. Then take ~30-40GB for OS/apps/packed in BS/etc. That takes you to ~400GB OOB. So, if games average 40GB, you have room for 10 games (or more likely 9 since saves will take some space, and probably some temp/crap space usage over time). Add in some digital music downloads, app downloads, a couple HD movies, and that size gets considerably more cramped. TBH, this is probably even more of a problem for Microsoft given their tendency to bloat the hell out of things. I recently installed XP SP3, Libre Office, Firefox+Chrome, Foxit, AV, Firewall, etc, and the entire installation was way under 10GB. Recently installed a clean system with Win7 64 SP1 on a 120GB SSD for someone, and 36GB was consumed by Windows alone. Just for kicks my Ubuntu box with a generous install (left most things on defaults), and it's well under 10GB as well, with Linux equivalents to all of the above.
Given that both start out at 500GB, and drives are pretty cheap, I guess I'm happy they stuck with $399 and a 500GB. 1TB would have been awesome, but I'm not going to complain.
You're right that this WILL crop up in the forums fairly quick though. I know way too many people that simply refuse to uninstall old games for a long time. A busy household with multiple players could even run into it with everyone wanting to have a couple of their own games installed.