I've tried setting up different accounts for each game I buy, but switching between accounts crashed my computer. Ideally there would be a steam launcher launcher where you pick the game and it launches steam into the correct account for you. It sets any settings/preferences as well. Each game would be installed in different locations on the file system.
If I have 10 games, I have 10 accounts. I play on one pc with one account and my son plays with another account on his pc. But then I can switch to a different game my son was just playing by using his account after he stops and he can play a different game on yet another account. All rightly and fairly paid for.
As it stands, I've bought a couple games from Steam competitors just so I could for sure play them when my son is gaming.