Dunno about competitive to the degree he is talking about - those that do that tend to be self-contained clans anyhow - but if you are playing with randoms at higher difficulty setting it is annoying when someone hasn't got a reasonably well put together setup and/or ignores what the other 3 players are trying to achieve.
Personally I find the game much more enjoyable post level 30 in coop - unfortunately they've not really given it the City of Heroes type treatment where both solo and coop were equally enjoyable at end game.
Oh yes, playing with Randoms is a recipe for despair (in all titles), in TD and many other games , I ONLY co-op with friends and family. Absolutely no randoms ever.