If everyone plays NC, NC will have no one to play with. If "a lot of people" join the NC, so the NC is overpopulated on most servers, again, NC won't have any fun gameplay because they will just zergstomp everything. In the end, this breeds bad gameplay for everyone. So why are you asking for this?
Well as long as their pop doesn't exceed 60% on any one continenent ... everything's usually fine. Being 2v1'd is their natural state and even with a pop advantage, their general incompetence ensures victories don't come easy. If anything the NC needs more people just because it doesn't often meet the quantitative TR + VS farming demands put on it. Invest in NC Directive Bonds!