if you want to sit back and ping people with high staff damge u can do that in the sets you mentioned. but if they close with you you are screwed.
generally pvp isnt about max damage. its about a balance between damage, survivabilty and sustaining resources. I use a set like u mentioned for pve dps and occasionally for a change of style in pvp keep defence etc. In pvp tho taking a small damage loss for MUCH more resistance and more recovery if u use sets like magnus seducer and healer is a no brainer to me.
stealthed heavy attacks followed by shock spam can be amusing tho on unprepared players.
Basically for blocking. Mitigation too, but mostly blocking.
Since 1.6 you can stack regeneration values sky high, so you can still cast a lot even without the light armour bonuses which you can recover a lot from using CP's. And PvP is all about balancing defense and offense, so heavy works well for DK's and Templars too since they do not have the escapability of NB's and Sorcerers.