So competitive questing would be optional in multiplayer, but mandatory in single player?
If it was optional, it would be the same amount of work implementing and testing, but almost nobody would use it (party members leaving at unknown plot points, possibly taking equipment, supplies and quest items with them, and placing the relevant quests on a timer, so you have to race if you want to be able to do it the way you want).

Any game I've played where a party member leaves has triggered reloading the last save, replaying up to just before that point, and then removing all equipment, etc. Adding in quest sabotage if you don't do them fast enough wouldn't make it more fun, IMNSHO.