Thats technically not part of a chain guide. The idea was that by having the chain line above the main pivot pedaling forces wouldn't be affected by suspension. So by re-routing the chain up with that little roller in theory it eliminated pedal bob, the Trek Session 10 used pretty much the same thing.

 It's a clue for me, too - it's interesting because you don't see that on every BB7 (I don't have one, too).

Proper chainline (achieved by correct BB axle lenght and frame pulley spacing) "keeps the chain in line with the pulley" itself, so this plastic element is not necessary.

In my opinion, it's made to keep the chain from rubbing the tyre (but it's just a guess) while riding continous bumps, jumps etc. (when chain is flying around).

 The black plastic piece that is within the purple circle behind the chain. Attached to the link plates. You made a comment in your first post, saying it is not a chain guide.
What do you call it then???

 Tell me what you call the black plastic piece before the pulley? You seem so technically inclined I am trying to find one for my BB7.

 I think its just a roller of some kind that's all part of that design to re-route the chain, sorry I don't have a better answer for you.

 Do you mean the black roller the chain glides over, or the back plate of the chain guide that is right above the bash guard?

 Wrong answer!! It's not a roller, rollers move. It a chain guide. It keeps the chain in line with the pulley.

 its a chain guide, you could name that a chain slider too