Isn't there a rather straightforward answer to this? Why not allow someone that dropped out due to an error (instead of mapping out or logging out) the option of jumping back into the team as soon as he logs on again? If he or she does so right away, the team will not have moved onwards too far in a mission or quest (or may even wait a little if he or she is a friend), so that player can catch up. In pvp, the battle may not be over yet (especially in a GvG). I am sure that people with less reliable connections would be very, VERY happy with such an option. And even people with good connections get errors or crashes sometimes.
Would this not be a great improvement? I can think of some objections:
- It may not be technically feasible (maybe due to the instanced world thing)
- It might be abused (for example, in pve, a player might force and error, log in with another character, farm a bit, and log back after 30 minutes, just in time to benefit from the team completing the mission, or in PvP, a team could partially log out, log back in on command - using vent or TS - and 'surprise' the other team or something like that.
I am sure Anet could think of ways to prevent these possible abuses (I can think of a few, such as a time limit, a 'permission to let xxxx rejoin' button for the leader, automatic DP, etc. There might still be some downsides, but would not the upside be wonderful?

Sorry if this has been answered already. I could not really think of a term to use for a search regarding this topic.