Everyone puts 3/3 into Improved voidwalker because I assume that while questing/grinding it's the de-facto best demon to level with.
But as far as I can tell it's subpar to the imp in pretty much every conceivable way. For starters if it tanks anything more than 1 mob I'm stuck funneling health into it. Say I tank one mob and it tanks another (which is what I do with imp). It's damage is subpar and if the mob is feared / runs away it risks aggroing more (and doing less damage since it has to move > aa > move etc). Also since the void walkers damage is almost negligible I feel like my down time is greatly increased since I have to expend more mana per mob and thus life tap and eat/drink more often.
Meanwhile with imp I can set it to attack one mob, while I attack another, I set up dots on my mob fear it and then set up dots on the imps mob, fear it and have me and the imp focus the other mob down and then return focus fire to the other.
As far as I can tell the void walker is pretty much only useful for an "oh shit I pulled too much better let him tank as much as possible before sacrificing him while running for my life" situation. Which granted it does a lot better than the imp in that regard but the slower clear times for pretty much everything makes me wonder if an occasional corpse run is preferable to the void walker.
I mean the icing on top is that I can solo elites that are my level with imp through good use of fear and I cannot do that nearly as effectively with the void walker.
Edit: The only thing I can think of is that 30> mobs may start 1-2 shotting a warlock? Or the pvp application of sacrificing the void walker makes it desirable.