Alacrity isn't good for one reason: It takes away from accuracy. Accuracy is pretty important for a merc, because your unload is actually your best dps ability. You need every tick of unload to hit. Currently, the merc is broken in pvp anyway and no stat change will help it. What they need to do is make a "cover" type stance for the merc so some of their skills can't be interrupted. That way they can compete with the awesome ranged class the sniper is.
No, it can take away from surge instead.
But more alacrity isn't worth it, it is a very minor speed increase, you can get unload down to 2.5s from a 3s cast by stacking up the alacrity and that's nothing worth having.