Your quoted numbers are misleading...I don't know where you got them, but they are most certainly not how many people are playing online....maybe you are just talking about number of people logged onto steam and are playing Sins, that would seem very reasonable....on a good night, you might see 300 or so on ICO, and most weeknights there's barely over a hundred if even that...
Last night we spent what seemed like 2 hours just trying to get enough people to play a 4v4...while that's a little extreme, larger games can still take 30 minutes easily to set up: getting enough people, picking teams, telling people not to play VR, trolling, cafe conversation, etc etc....
Most games are decided by drops (either by people quitting or minidumps) or by noob placements/stacked teams...you play Sins on ICO to have fun, not to get a good record or climb a ladder (btw, MP ladders and matchmaking don't exist and likely never will)...there are definitely some very good people out there but its tough getting large games together that have only skilled people...
The minidumps that occur have nothing to do with steam fortunately, they are the result of Sins itself (and maybe ICO, but probably just Sins)...while that sucks, at least there is hope the minidumps will be eliminated by the devs (who do try to update frequently)...if you minidump, tough shit, game's over for you and your team is out a player (well, technically an AI replaces you)...happens a lot and it's just something you get used to....
I love Rebellion, but I also am into modding, playing LAN games with friends, playing SP, and playing non-competitive MP games...competitive/skilled MP games is only one part of my sins experience and since I don't care about my win/loss record if shit happens, then shit happens...
Most people complain more about the game balance than the game stability...Vasari Rebels are pretty much banned from any serious MP game, and Vasari Loyalists are grossly OP by late game if they start as eco player...there are other issues (Corsev, titans in general, etc. etc.) as well....
If you really are hesitant to buy the game, I'd wait for the next update (v1.04) before deciding not to buy rebellion...the most glaring balance issues may be resolved by then, and the game stability may improve substantially...hell, even some of the old guard pros may come back to MP....