How would they fend off the incoming rats without a weapon? If you tell me to replace one of the miners with a gun I will slap you because mining drones alone are ineffective compared to the miner, specially to an Alpha where skills play a huge role on drones.
You said it yourself, mining boost is tiny, so you either have a dedicated person babysitting you to protect you all or just watch the ships slowly burn because you only got one gun to kill them that suffers from lack of tracking when drones could do the job better or waste precious mining time by warping out, getting in combat ship, warping back, killing rats, warp out, mining ship, warp in.
We do not even know the amount of rats that will show up in Resource Wars, we only know that difficulty will scale up the higher the lvl of the site, up to Battleships.
Even with the same numbers that the already existing Civilian Mining Drones have you would be trading effective drone damage for ‘some more ore’, making a huge risk on how your gun will most likely not be enough to fend off incoming rats. You are not aware of what will come to attack you, you are not aware if you need defenders, nothing. Try to tell me that one gun is enough, I dare you to come to Caldari space with just one turret in your venture, see how that works against the rats ECM.
This is a terrible habit to use the small drone bay to stick in mining drones when it is already tricky enough to fend off rat frigates in a venture with a flight of drones, if it had a big drone bay to hold up to two flights of drones, sure go for it, but you are better off using combat drones with a small drone bay
Besides, a venture can only field 2 drones, which makes things worse, both on the mining yield and on the damage field, but two hobgoblins are still far more reliable than one gun trying to track the orbiting rat.