Can I create one steam account per game?

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?

There are other reasons to do this:

In my house, we started off with one steam account. As my son got older, he enjoys playing games on the account, but that prevents me from playing ANY of my games on my other computer while he is doing so. Why should I be prevented from playing Civilization 5, Skyrim, or Fallout 3 just because he wants to play Half-Life 2?

So I started making new accounts so that we could both access Steam's library. (Personally, I think it's ridiculous that I'm forced to do so, considering they are different games.)

I like the simplicity of how they run things, but you can see how it would be a real problem for someone like me.

You dont need multiple accounts to play different games on the same computer. I have access to two computers, both of which have steam on them; so somtimes when i have a friend over and we both want to play one of my steam games one of us will log onto steam and then go into offline mode, the nthe other will log on and voila. Two steam games at the same time.