I had one game a week ago (during this last Nova week) where we were matched 4 assassins and 1 specialist vs 4 assassins and 1 specialist. Our specialist was Abathur and theirs was Hammer. Both teams had Zeratul (I was our Zeratul) and Nova, both Novas were not skilled. The thing is this Abathur was going for a siege build, so he died a lot getting discovered BUT he was never buffing our team, we managed 4v5 quite well and the match dragged for quite a while, I still have the replay and I can send it to you whle the game is still not patched to being incompatble with the replay. But the thing is he pushed a lot, like really a lot and later we cleared the way to core before taking boss. It was such a good game!
If I was playing Illidan maybe I would find some use in Abathur but with Zeratul not much except for shield while running away, the hat is a dead giveaway while in stealth since they shoot. It's like when Rehgar players put their lightning shields around hidden allies, it's stupid.