I think this pretty much sucks. Especially on boss monsters (e.g. Roc in coerthas).
When there are already other FATEs nearby, people will just leave once they have dealt enough damage etc. and get gold anyway and can already start the next FATE.
Other people can leave as well, resetting the whole FATE or killing the boss with 3-4 monsters.
I think FATEs need some system like if you haven't participated(dmg,heal,etc.) for 60 seconds, your whole FATE progress is resetted. People will rethink leaving FATEs midway then and won't leave the tank and some others to kill the last bit of the boss dealing pretty low damage in general ._.