Lord Sessadore said:Also: can you think of any possible way that a rogue would not have surprise (and therefore combat advantage) against an enemy that hasn't acted yet, other than Alertness? If there is no such case, why even include the ability? It would seem to me that First Strike is made to specifically counter such abilities as Alertness.
The encounter didn't begin with the PCs ambushing the monsters, and the rogue beat everybody else's initiative roll?

You're right that Alertness doesn't negate Combat Advantage, but Combat Advantage applies in a lot more situations than the surprise round.