Currently warlocks sometimes put Dark Intent on Survival Hunters who DoT-crit a lot. It is also put on Druids of the Boomkin and Resto flavors. It's a nice buff. You'll fire a fair amount more. A warlock will usually want to put it on someone who crits a lot. If the boomkin in your group doesn't crit as much as you, then you're a good choice. Both SV and boomkins benefit from it. Boomkins may benefit more if the number of DoT crits is comparable. If healing is low however, the warlock should put it on the resto druid to help them. Generally it wont hurt the warlock much.
The new PTR notes for 4.1 show this language:
"Dark Intent: The friendly target of this ability now receives 1% (stacking 3 times to 3%) periodic spell damage and healing bonus instead of 3% (stacking 3 times to 9%). The casting Warlock still receives 3% (stacking 3 times to 9%)."
Note the words "spell damage". Spell damage doesn't look to help us at all. It's also not haste. So... if this isn't just a typo, all the previous Dark Intent calculations are out the window.
That said... it is not at the time of this writing... what is shown on wowhead's PTR tooltip for Dark Intent. Yet.