When I play stamina DK in trials, I feel I don't support the group in any way. Sometimes I used the magma shell ultimate, but it's just too much of a dps loss.
Other then that, I think noxious breath is the only thing that can support the group.

To mention a few examples:
Magicka sorcs provides with lightning damage, which is off balance and minor vulnerability.
Magicka DK have engulfing flames, which increase all fire damage done to target by 10%.
Templars have power of the light and nova.

Stam DK's can use noxious breath to reduce enemies physical ressistance, all though it mostly works on adds since boss already are debuffed with major fracture.
Maybe this skill could work like elemental drain does with the minor magicka steal, but giving stamina back instead. It would give stam dk a reason to be in a group as well as the stamina characters in general got some help.
Noxious breath itself in pve might be fine in some situation, all though since most players are magicka atm, the major fracture are not that usefull. As for selfish damage, the noxious breath is the lowest damage dot in my rotation, but gives a bit more poison status effect.
It is still a decent skill due to AOE and long range, but I still can't find a proper reason to use it atm.

Any thoughts of other experience?