Are one-off events a good use of developer resources?

While the numbers are far from being a perfect measure of what is going on, The Nosy Gamer's weekly XFire stats are the best we have to look at activity in various MMORPGs week after week. And this week's numbers show something already previously observed: While Guild Wars 2 activity spikes on weekends with world events, the activity drops to below pre-event numbers the week after. XFire detected 20K hours spent playing GW2 the Sunday before the event, 26K during the event, and 18K after the ev

World of Warkraft...