Honestly, at this point, i dont really think it would make any difference having 100 or 1000 persons online. It was very hard when delve 18 man was the main activity, as making a party for that was always a very ungrateful task (shouting 1h+ for brd or sch). Now with everything low man, any activity can be readly fulfilled with persons in 30 mins tops. Any ls can get 6 persons so so geared and do a normal battlefield, do a beaded run in delve with good chances of winning.
The only pro i can see having more people around is more economy activity, but also more competition for stuffs like dynamis, ark angels, vnm, mobs in exp/cp parties. I would go as far to say most people playing dont really care if the server merge or no. This was diferent pre ark angels, but nowaday, if youre bored/afk/have nothing to do, you cant really blame the game. It already did what it could to keep you busy with few friends.