Defense drones can pay for themselves in a hurry if they shoot down a couple of missiles, and odds are you have drone parts to burn anyway, unless you’re doing some sort of heavy drone attack strategy. And they are pretty much the ONLY defense against missiles, which are quite mean. At most points of the game, one well(or rather, poorly) placed missile hit can mean the difference between a total cakewalk and a fight that actually costs you hull and effort.
Frankly, I’d argue that the anti-personnel and system repair drones are the more-or-less useless ones unless your ship is badly under crewed and you need to make up for that. System repair DOES trigger some nice blue events, but most of those can also be “blued” by appropriate crew members, so the usefulness of these drones varies widely depending on whether you get decent crew or not. On the other hand, there is basically no other defense against missiles (except cloaking, which will help but won’t do nearly the job of a defense drone.) Sure, you can always blow out the enemy’s weapon systems, but they’re probably going to get at least a shot or two off first, unless you’re going for some crazy alpha strike potential, in which case you’re probably not paying power for drones anyway.
Attack drones are pretty good, though beam drones are kinda meh for me. They do lots of damage, but only in specific circumstances, so I generally won’t pay for them. Actually, I try to avoid paying for drones at all. Defense 1 is about the only one I’ll actually pay scrap for. Everything else I can mitigate some other way, but missile damage ruins my day.