Its good, but no its not mandatory. It is, however, if you plan on doing a close ranged build.
In Normal and early NM, you can afford to go in for HS on a ranged build, but you get to a point where that is no longuer viable. That is why people clamor that Spirit Walk is necessary, cause it lets them continue to go in. Frankly, I think its more for mid/close ranged builds (plague bats/explosive toads/etc)
Personally, I run with neither and i am doing perfectly fine in act 3 NM right now. Build i use is this:http://us.battle.net/d3/en/calculator/witch-doctor#fVTPYj!aeU!YZZZbb
Main damage is Zombie Charger with Explosive Beast runed. The damage is borderline amazing and the range is great, keeping in mind its an AoE. Frankly, it feels like having a rocket launcher (splash damage even means you can aim at a wall near them if you wanna stay around the corner while they are distracted by your summons). Your sacrifice is your big burst, and can blow entire packs on its own if you time it well.
I use my pets mainly as decoy and Sacrifice fodder. Get some gold range gear for Circle of Life so you dont have to get close to mobs, you dont need that much. You may want Devout thingy on Confusion so you can get more dogs (and more explosions), but I prefer Mass Hysteria for the assured CC. That being said I plan to switch it out for Angry Chicken the moment Blizz fixes the disconnect bug on it.
Anyway, beyond my stoned ramblings at 3 am, my point is that its not mandatory. Its good because in closer builds it does allow you to get close enough to Harvest and still have a way out. It is also an amazing health OR mana regen tool. But you can do without if you plan a longuer range build.