The dexxer plays a far different role in both pvm and pvp, because of this they're often overlooked.
You're right that each situation will dictate a different role, but as much of our pvm involves bards I'll use that as an example...
Dexxers are well suited for baiting mobs for the provokers...that is running in first to get the mobs attention (I go in warmode and attack each of them, as that gets their attention instantly
)...this makes it much easier for the bards, as they're not being chased while trying to get their provo off....this is especially true when there are multiple mobs on screen (think Ophidian Lair, heh.) With multiple mobs, I usually run circles around the bards (tho not letting the mobs get too close to the bards of course) keeping the mobs distracted until the bard's provo timer is ready for the next.
The benefit to this is not only that it means one less thing for the bard to be worrying about, but also dexxers are the best armored in the group, meaning the occasional hit will not be as devastating as it would against a bard or mage....Also, with bandage healing, you can continually heal while moving, allowing you to stay at full health without stopping and taking more hits than necessary.
After the creatures are all provo'd, the dexxer contributes by keeping each mob on par with the others health. (Ie: two lich lords one at 3/4, one at 1/2, jump on the 3/4). This makes provoking much more effective.
What's nice about this scenario is that it often leaves the dexxer in a more relaxed position. Once the creatures are provo'd and you're just keeping them on par, there's nothing stopping you from pulling away from the pvm if need be....such as when pks roll in, heh.
For group pvp, the dexxer plays the tank so to speak. You run in, always staying on them, putting the pks on the defensive a bit so that the others in the group are freed up (as they're not being bombarded with spells when the pks are running and spamming "In Mani", heh) to cross heal and dump (when the moment is right). The best bit of advice for a dexxer like this is to always stay on the mages, even when at low health....If you back off when you're low they can give chase, as there's now nothing stopping them from prepping a dump. Stay on em even when you're low and you'll be disrupting their offensive spells, putting them on the defensive to heal....just stay on the offensive and keep 'em busy. Always keep your bandages going...if it seems like the bandages won't kick in fast enough, drink a heal to keep you going until they do....Also, be quick on the cures (as poison will cause your bandages to cure....a missed bandage will ruin you.)...and keep up on the stam pots, as they dictate how fast you swing.
Not the most detailed advice or tactics, but I hope that helps.