question about defensive sets

What kind of def set should I run on a Dervish? Never played a derv much, even in PvE, and I'm just not sure I've got the right weapon sets for PvP.

Well, def sets are not so important for melee professions, cuz most of the time you wield your weapon(scythe in your case). But it might be useful to take -20% blind shield (effective against air eles, especially if you have -20% blind rune in your armor) and switch to it when you expect blind. Also -20% cripple might be useful in GvG to reduce time of Archer's pin down(if you failed at dodging it) and ranger's Crippling Shot(or pin down in case of Magebane build) + vs. Fire shield for GvG if fighting on Warrior's/Wizard's/Hunter's isles(Also u can take shields with +10 vs every type of damage but i doubt if you'll use it on your derv). For the weapon i recommend you to take Furious/Crippling Spear of Fortitude/Defense.

Doesn't the "reduced crippled duration" bonus from the shield apply only if you were crippled while having the shield equipped? Getting crip and switching to the shield then should have no effect on the duration of current crip. Yes? No?

of course you need to have shield equipped before u get crippled for reducing crippling effect. So, if you expect cripple, you should switch to your -20 Cripple Shield set and still try to dodge it and even if you fail to do it(well... if you don't know how or there is two archers and you successfully dodge first archer's pin down but get crippled by second archer or whatever) you still have cripple reduction so it won't be long(well, not so long how it could be)

P.S. -20% Cripple shield is also effective on Burning Isle when going through lava.