Drones serve a purpose... A very, very, VERY niche purpose.
A purpose that is almost never seen or even achieved in 95% of matches.
I believe their largest drawback is their resource costs.
Each drone is equivalent in cost to that of a T1 transport.
A drone's best operational time is very, very early into the match - for expansion.
That is quickly offset by their return value.
Which is close to none.
Another drawback is that of the awkward time you need to have your ACU just standing there just to get another 5 BP that would otherwise get shot out of the sky by someone dumping like 5 mass per second into Interceptors.
And the 2nd drone is basically as useful as the first... meaning it isn't.
But, if you manage to pull off a play with them - they're almost worth it.
Good luck with that, though.
~ Stryker