[Taera]

There is another use for them - when you reach ships as large as a destroyer or a light cruiser you can put more than one mining component.
Of course only one ship can work on a place, but this ship can have as much mining components.
If the asteroid value is 120+ is makes up the maintance cost, especialy is you use Ion Engines lvl. 3 <- simple but cheapest engines.
Decreased maintance by race settings and facilities also helps.

Actually for people playing it "the hard" way it might be the only way to make it up on the minerals in the shortage monhts.
Ice/CO2(my settings) mineral rich planets are a rare catch and two or three mining ships arent rare in my fleet.

Another thing, for normal races it might be logic to mine organics instead of building plants to harvest them - they hardly use it, especialy racial traited ones - crystalline and temporary and psychic etc.
Organics on the other hand might just mine the radioctives 'cuz they barely need them (who needs fusion and stuff to power a claw???)