Edit : Btw prinz dots and hots take full advantage over +dmg/healing items. But you mean since the duration runs out before the dot is finished it doesn´t add?
hmm, well, this is how i understand the +dmg to work, from a post made some time ago by a blue on blizz forums
for sw:p and dp, they're insta casts, so they count as 1 sec casts and get 1/3.5 part of the +dmg stuff, which is then spread out over the entire duration of the dot...so sw:p would get (1/3.5)*175/18=2.7 dmg per second extra from activating the talisman, not very effective ( some of the wl's dot have a base casting time so they get a bit more effectiveness out off it)....
same goes for channeled spells i believe, which are also not extremely effecient because the spell usually lasts more than 3.5 seconds yet the 175 dmg is spread out over the entire time of the channel...
this is how i recall it from a post made by blizz at some point, i couldn't find the original post on the forums, would help if someone could find it :)