Last time, I extolled the virtues of mob mode. Now imagine the following scenario. Your character is walking down a street and bumps into a friend. This other PC is one that your character has spent a lot of time running with and they know each other well. Naturally, your PC greets him/her and gets the following response:
That is an immersion breaker right there. There is an inherent problem with NPCs communicating with player controlled characters. Generally speaking, you have two options. You can use canned conversations, or go the chatbot route. Canned conversations are of course canned and mostly suited to utility NPCs; quest givers and merchants. Chatbots have difficulty with context and are prone to sounding deranged. Both of these break immersion.
There is another issue with mob mode raids/adventures. If the player assigns the PC to a friend as a pet, the owning player is no longer in charge and the temporary controller is. What if the temporary controller takes the character somewhere that the character would not willingly go? What if the temporary controller engages in forced RP with the character? How much control is ceded to the temporary controller? What happens when the temporary controller asks the PC to do something that the owning player does not approve of? How do you define the rules for handling such situations?