Guild Wars 2

Are thieves really good for wvw?

Shortbow/Dagger-Dagger

I don’t WvW very much, but in PvP they can be a right pain as they’re often invisible half the time. So I’m guessing yes.

But really, I think any class played well will be really useful.

they are good at ganking and being generally annoying. they are bad at everything else.

Shortbow is ok (can be used in defense and offense while sieging a keep), Dagger-Dagger can get you in some trouble if you don’t time it right. Also, as well as any other class, you need to adapt to the roles you want to play.
Scouting for the server? – Get as much stealth and heal as you can get to roam behind enemy lines.
You want to zerg? Conditiondamage and AoE are your friends, heal on each hit is preferable.
You want to defend? Again go with AoE and conditions.
You want to go in small groups, 1on1? Get health by hit, cloak yourselve a bit and deal direct damage+conditions. Know when to stop and retreat (eg Confusion).
Thief can be lots of fun in WvW, but it can get frustrating if you don’t know how to. Just play and you will learn whats important. There is no ultimate concept for any class in GW2.

Cheers.