Steam – Does Steam Track Network Usage in Megabytes or Megabits Per Second?

It doesn't cost nothing - it costs time and convenience to create different accounts and log in to them all separately when you want to change which account you are using.

I searched through the User Agreement. I think you're correct in that this is not specifically prohibited. Similarly, using multiple accounts to purchase multiple copies of the same game and multibox is not specifically prohibited (to my knowledge).

If you are the kind of person that would actually want to play different games at the same time, it does make sense. However, I cannot think of a legitimate reason to actually want to play multiple different Steam games at once. I have a hard enough time being good at any 1 game I'm playing.

Should you be doing this? In my opinion, no. I use the community-oriented features of Steam all the time and the Cons you list would affect me greatly. Having multiple accounts would be a tremendous inconvenience for me, and I don't care about any of the Pros you list.

Exceptions: There may be some games where it would make sense to have a separate account for just that game. I hear you do a lot of sitting around in games like Eve Online; perhaps you would want to play Bejeweled while you wait for something to happen?

A VPN takes over the entire OS' connection. It is program-independent. As such, a VPN will work.

Tor is more doubtful, because I don't think Steam supports a SOCKS proxy out of the box. There are programs out there that force a certain program to go through a SOCKS proxy though (which I couldn't find on a quick Google. I'll have a look around).