Full DPS teams can actually be very strong I've found, even on higher ratings. I myself (as a rogue) play with another rogue... our main strategy is to completly burst 1 enemy to death ASAP while keeping the other blinded (if he trinkets my companion blinds again), then running off, restealthing, and starting a 2 on 1 on our terms. We've even beat warrior/paladin teams like this by nuking the paladin into his bubble, running off and restealthing, killing the pala, and then killing the warrior.
Common sense would say 5 DPS in 5v5 is completly useless... Technically it is, since surviving the initial burst guarantees a win. On the other hand, nobody ever suspects 5 DPS (let alone 5 rogues), and if you make but the slightest error during the burst you'll have lost at least 1, probably 2 of your teammates. And 3v5 is hard to win regardless of the classes in play.