It would also be self-limiting ... you could only farm bots for rank if there weren't any actual players about, but if "everyone" tried that there'd be actual players about.
This is to an extent an argument against putting too much work into the bot quality - if they're actually successful at getting players to the game, they won't see much action themselves. Ones which bounce off the scenery, ignore the powerups, and otherwise do a good impression of a beginner ... are probably sufficient.
Yep. At this point even dropping the regular NPC AI into CQC is much much better than nothing.