Only when the devs need it for testing. Big new patch, change of server (eu1_new for example).
Its fun once in a while to have 200 players in a server, but to be honest i got bored of it after 30 minutes, might as well be 50v50 as far as im concerned.
XP should not be a reason to play or not to play, grinding once in a while is ok, but focus on keeping the game fun. Regular XP events give a temporary boost to playercount, but will encourage grinding and have a bad influence on the activity over time.