So it looks like TF2 is adding bots, I am not sure of how this will work but I trust that valve will make the best of this. They are going to use the same AI system as L4D used. 
Quote from the official TF2 blog
 " Team Fortress 2 is an excellent "sandbox" for explorations of this sort, and we've been quietly doing so for much of this last year. Some of the results of these explorations are TF "bots" 
 
They are testing them now and could use your help. If you are interested got to 

Well, it doesn't take anything away from the game, so yay all around.

Since its something people have to enable on servers and most servers I play on wont ever use, I dont really care either way. AS Rjay said, it takes nothing away.

I guess that means we'll be able to play offline, and that is really great, but how well will it actually work? It will require a really complex AI system, I suppose.

I actually joined a server a couple of days ago that I suspect was comprised entirely of bots.  It was on a Payload map and I honestly seemed to be the only person attempting to do anything with the cart.  I said some things in chat to see why nobody else was pushing and there was no reply, on a 30 player server nobody said anything in about half hour of play.  Honestly though, they seem to play really well, besides the whole not pushing the cart thing I could not tell them apart from normal players.

I remember the bots in Counter Strike Source responding quite realistically through voice chat.

I would have loved this when I first started off playing TF2. It was quite daunting, and wanted to practice on my own for a bit first. But now I want to play with real people, not bots. I suppose it'll be fun playing games with a small team of human players against a large team of bots. Me and a couple of mates used to play CS:S against a large team of bots and it was great fun. I suppose it all depends how well the AI works.