Soul harvest is designed to be used by magic toons, incap strike by stamina. Note the type of damage they do...disease scales off your max stam, magic off your max magic.
However, soul harvest is one of a handful of skills in the game that offers something that's really quite distinct - 6 ultimate per kill. That's huge. On my stamblade, I've moved on to other ultimates to replace incap strike. The dual ultimate is an amazing replacement. So for this character, because I was never using incap strikes, I put it back to soul harvest to use as an ultimate generator.
And if you're a magblade...soul harvest is amazing. Put it on your attack bar to generate ultimate, then put the strongest ultimatemyou have on your back bar. Ka boom.