How does Abathur contribute to team experience?
Locusts do not grant the AoE experience like heroes do for just being nearby the death of a unit. Instead, locusts gain experience whenever something dies after it has been damaged by the locust within, I believe, the last 5 seconds. Similarly, your mines also grant experience under the same conditions.
Abathur, in theory, can cover all 3 lanes for their experience because of his long-range abilities. 2-3 mines can easily blow up minion waves in one lane, a pack of locusts can be in another, and he can be nearby the third lane or even cloned at this time.