First off, what are the advantages of having Ai Bots in the multiplayer aspect of a game? Well, they allow new players to that game or otherwise novices, to get familiar with multiplayer game-play without the general difficulty of more experienced online gamers (or ridicule from online gamers, either). What I mean is, they can play with these Bots in custom games on some easy setting, which will give them experience on the multiplayer maps, get familiar with the weapons, and get a general feel for online play. Then at their own pace, increase the difficulty settings of the Bots to become more skilled themselves. Whereas say with Campaign, which does not really prepare a person for the multiplayer experience.
Other advantages of Ai Bots include being able to play multiplayer game-types offline when one has a lack of online connection, or otherwise a poor one, which would greatly diminish the positive aspects of online play because of lag. Bots can also be used to allow players to play without worrying about opponents using cheats or exploiting bugs in the game. Furthermore, Bots can be used online to fill in spots of players whom have lagged out of the game or quit.
Perhaps an ultimate experience with Bots would be a specific play-list in match-making that would be deemed ‘Humans vs. Bots’. You go in with a team of 4 of your friends, then you are matched against 4 Bots. You go in with 2 people, then 2 other people whom are searching in that play-list would join you, then the 4 of you play against 4 Bots. But now say in that play-list one of your teammates lags out or quits. Then simply one of the Bots would drop out, thus always having even teams.
This concept of Bots in a multiplayer setting is not without its disadvantages or flaws of course, such as gamers exploiting the Bot fill in feature in match-making by intentionally quitting out of games so that more skilled Bots can take their place (which will say help rank up their friend). And although skilled Dynamic Bots can be designed and implemented into a game (such as RealBot for Counter Strike), a game designer may not want to use a considerable amount of their budget on such an aspect of the game, and so may settle for a cheaper (and thus less advanced), version of Ai Bots.
Overall though, the positive aspects of having Ai Bots implemented into the multiplayer portion of a game would seem to outweigh the negative. And so, at the very least something to consider.