Ye. or make it so that it only works for players who are contributing to the party (ex: blasting / healing / buffing/ doing something). This way leechers who just sit and do nothing won't gain from it and it would help priests out.
Interessing, but how to do that. Something that look out for inactivity and you lose ur double exp if u passed the inactivity time? Simple macro to cast a spell will get ride of that.
Also, feature like that keep drinking memory.