Imagine this: you're on Dragon Shire, when you notice that there's a single enemy hero guarding the bottom temple. Should be easy for you if you bring a teammate along right? Then you walk through three mines, dropping you to half health the enemy hero's suddenly wearing a spike-shooting hat.
That's what Abathur does. He's not as strong as another hero physically being there, but he can be anywhere on the map as long as he has something to Symbiote to.
As an Abathur it's about positioning, really, you need to be in the right time at the right place. I very honestly think that Abathur is the biggest culprit when it comes to enabling lousy players: they think just because they're doing something, they're helping.
Anyway, some advice: