Well the thing is that most of the latest bots make it so hard to tell the difference between them and people doing the same thing. If you or I went into a game and just killed off all the monsters, picked the drops up, and repeated the same thing then obviously we would not click in identical locations every time. But the bots now have randomizers that make it so they don't either.
Furthermore, the bots are able to go back to towns, buy supplies, and use them. They can even sell items that you do not tell them to keep. They can cast protective spells and there were even "Chickens" that made the player instantly drop from a game if the health got too low.
They are able to detect monster types (staying away from monsters that the character can't kill) and identify the drops and compare them to a database the user sets (to decide whether or not to pick it up and sell/keep).
For all intensive purposes, most of the bots are indistinguishable in action to you or me playing. The difference is that these smart-bots have to send packets without action (e.g. the mouse never moves, it just send packets saying the mouse clicked somewhere), and that would be the only way I know of to detect the bots (if even that).
Ultimately the question comes down to one thing: Are the items worth the bots. I don't mean to try to instill panic in saying that 3 days after the public release we will be overrun with bots, but any skilled programmer who finds a need to harvest items and who has the time can make the bot. Something as simple as a code randomizer that makes you enter in a 3 or 4 digit code to start a game (as previously mentioned) would drastically reduce the viability of bots. But even then alot the hackers (the people behind the bots) were tossing this around about D2 and coming up with ideas on how to circumvent it.
From what you all have said about how the game works, I do not suspect that bots will become as widespread as they did in D2 (since that is the extreme example), but I definately see a possibility for some private bots.
EDIT- I just realized I say "well, the thing is" way too much
