I know this is done in Counter Strike, I think it would be worth implementing to see how well it worked.
To the Director, if you just had your teammates with high BPRs leave, the game wouldn't give you bots that were ungodly in comparison to your friends, it would defeat the purpose of your friends leaving. And if you signed in guests and signed them out mid-game, even if you weren't kicked out of the game, you would still have been matched up against players with BPRs at the same level as the gamertags you signed out.
But I am quite skeptical of this as well, so if they did put this in the game, I would think that they should probably make a separate playlist for it, and maybe adding it to other playlists later on if it proved successful. I would also like to see a gametype similar to Black Ops' Combat Training, where you can play against bots. I think it could be great for warming up. Just realized Absolute Dog said the same thing, I'm still leaving it in my post though.