..and why Assassin was a bit overpowered last preview even before Factions.
Being overpowered is the way for many to love the class. And if you love the class you're going to buy expansion. "wow this class is great" is much better stimulans than "they are ok..." (for buying new chapter that is). That's the reason why Anet feels it's better to make class somewhat overpowered at first, and then before chapter is out, balance it.
I dont see any other reason. Let's face it, you cant miss how uber some of their builds are, especially considering Dervish is supposed to be better in pve. Well if they are this good in pvp atm, wow :P