If my maths are correct, Soul Harvest will still add nice damage even when you have good gear.
If your base Attack at level 60 is 1000, Soul Harvest will feel like a 57% damage increase.
With 2000 base Attack, it will feel like a 30% increase.
3000: 20% increase
Let's compare these damage increases to adding an AOE DOT instead of Soul Harvest. If you were going for a build that could do consistently high AOE DPS from range attacking once per second and mana isn't an issue, you would probably pick the skills in the following link starting with the top and moving down to the bottom:http://us.battle.net/d3/en/calculator/witch-doctor#RbYhZS!!ZZZYYc
Corpse Bomb is your main nuke, giving you 213% DPS.
Adding Burning Dogs increases your DPS to 321%, which will feel like a 51% damage increase.
Adding Groping Eels increases your DPS to 399%,* which will feel like a 24% damage increase.
Adding Humongoid increases your DPS to 451%, which will feel like a 13% damage increase.
Adding Rain of Toads increases your DPS to 489%,* which will feel like an 8% damage increase.
Adding Cloud of Insects increases your DPS to 518%,* which will feel like a 6% damage increase.
*Takes into account that Corpse Bomb will not be cast when you are casting this skill. Zombie Dogs and Gargantuan have no duration limit so they are assumed to be cast before combat.
So at 5000 base Attack, Soul Harvest is your 5th best damage choice.
At 4000 and 3000, it is your 4th best.
At 2000, 3rd best.
At 1000, 2nd best.
These conclusions also assume that you picked Vengeful Spirit as your Soul Harvest runestone, which does more damage than Corpse Bomb, making you not lose damage for a second while casting Soul Harvest.
This doesn't take into account passives like Provoke the Pack (which would fit in around skill choice #3-5 depending on other variables), but you get the idea: Soul Harvest will probably be a part of optimal PVE damage builds. When you first enter Inferno with gear farmed from Hell mode, Soul Harvest will be a big damage increase. As you start adding Inferno gear and your Attack increases, you will probably need fewer survival and mana regen skills, so you will still be able to fit Soul Harvest into your build.