Tobold talked a little bit about the companions from the new Star Wars MMO. He believes that it would be relatively easy to create a bot that could be a better player than any player and let these bots sub in for player roles. The idea is that although companies could easily create bots that could smash encounters in MMOs they do not because it would make players feel bad that the bots were so much better than them. Hogwash I say.
Certainly bots have advantages over humans. They can process data like "I am in a fire" and arrive at the conclusion "Gotta move" within .01 seconds. They can maintain a rotation of casting spells with 100% of normal efficiency while moving and they are never distracted by their kid crying, a hiccup in connectivity or being hungry. The problem with bots of course is that they are dumb as hell in ways that are mission critical. When the boss is casting a gigantic attack the bot is going to die because it doesn't know that 50% of the map is a death zone. When the group strategy is to clump up on command the bot doesn't understand that - or it takes a single tick of fire damage and wanders in the wrong direction.
The point is that if you program a simple bot to cast spells efficiently and not stand in a simple fire it can beat a terrible player quite handily. However, if you want that same bot to react effectively to complex or innovative encounters the bot must be programmed with all the strategies ahead of time. This would be utterly disastrous as it would mean that all the players could simply follow the bot and do exactly what it does - this has to be successful or the bot isn't functional! There is no possible way to write a bot that can react to interesting situations in useful ways unless you think that the bot can die and then learn to play better next time. Unfortunately this is simply not possible with the current state of technology - if you think a computer can look at an encounter and devise effective strategy alterations to deal with it then I challenge you to take the World of Logs code from any WOW raid wipe and devise a program that can analyze it and tell you effectively what went wrong with the group's strategy. You will fail.
The reason bots are bad at playing MMOs is that in order to be effective against anything but the most simple and repetitive challenges they need to have knowledge of the encounter ahead of time. Screwing up any part of that encounter programming would make the bot useless though and even if you were successful it would really kick immersion in the nuts; players would wonder why their bots were psychic ninjas and anticipated every boss ability on the first try.
That isn't to say that there is no place at all for bots in MMO play. Finding groups for things can be challenging and having a bot that tries to get out of fire and follows the player around casting healing spells can be great. Having a tanking bot that just attacks any nearby monster and fights to the death has its place. These mechanics could make filling 5 man groups or doing quests easier and give people options when their friends are offline but they don't want to play with randoms (or can't find randoms who want to play with them!). The idea that somehow these bots will be better at playing than the best humans is laughable though. We can see this in Starcraft 2, where the computer playing as best it can is a suitable challenge for beginners but extremely quickly becomes easy to defeat and is a complete joke to experts even though that game is so dependent on click speed and multitasking. These bots will only be useful for trivial tasks however as once an encounter really requires strategy or practice the bots will be useless.