If you buy a chair, why would you buy a second chair if for the same matter your housemate can sit on yours when you're not using it?Misinterpreted Cadiuro's post, left it here though since I like it.
Really, it is YOUR chair, you can do with it whatever you want, as long as it's not illegal. Allowing other people to sit on your chair is NOT illegal and is even considered good karma most of the time.
The sharing steam account argument is irrelevant imo. It's a nice side-effect of ingame accounts, but not the primary reason multiple ingame-accounts should exist. I have already stated the primary reasons earlier.