I feel that the use of bots is fine. There's no difference between a bot, who you kill 1 second after they respawn and a friend, who you also kill 1 second after you respawn.
<EDIT>Heck, your buddy is probably off doing other things, like using the bathroom or getting a soda. So now either way you have it, bot or buddy, they are both doing nothing except letting you kill them.</EDIT>
I would personally rather use bots, as it is more convenient, and I for one, do not have to bother my friends, who probably don't want to be bothered. I don't see a difference.
Lets take the achievement where you have to reflect 100 projectiles.
W/ HUMAN AID- Your buddy fires shots at you, and you reflect them.
W/ BOT AID-Your bot fires shots at you, and you reflect them.
The difference above is that with the human aid, you're spending twice the regular time to get the achievements, as your buddy will probably want to go pyro, and have you fire shots at him. So instead of taking 1 and 1/2 hours to get your achievements, it now takes 3 hours to get your achievements.
Also, its not like you can get ALL of your achievements with bots. Take "Burn Ward" for instance, this is easily do-able on a public server. However, when you create a server with bots, its pratically impossible (at least, I have not found a way to do this succesfully).
With those reasons, I ask you, what is the difference between using bots and buddies? Other than bots getting your achievements faster.
I for one, find bots to be alot more ergonomic. I.E. You get more for your money...or time, in this case.