The SLF bay slot is not that big a sacrifice given the tactical advantage the current combat SLF variants can provide.
But how big is that tactical advantage when mining? Other than the odd pirate that spawns alongside the player in the instance, mining doesn't really require combat so it is pretty much a waste to load up on combat gear beyond a few basic self-defence weapons.
For a miner, that SLF bay slot means giving up cargo capacity or limpet controllers, which does impact on overall performance.
In terms of generally making SLFs with mining equipment, I am all for it as long as our crew members understand how to use them. At the moment, big ships do suffer from pretty poor scaling on mining enterprises, in part due to poor scaling on mining lasers but also the terrible scaling on limpet controllers, and adding in an NPC-capable SLF would help to resolve this issue.
A basic mining laser SLF would be able to function as an additional pair of lasers, although if they adopted the wing-style mechanics of asteroids being per ship it would help greatly as it would double potential yield from asteroids. This would be doubly so if it were capable of holding a few tonnes of cargo or it's own limpet reserve. At a basic skill level, a crew member would mostly fly around chipping off fragments for any asteroid it can find within telepresence range, while the more skilled ones would be much better at tracking down and focusing on the high value goods.
Alternatively, an abrasion blaster SLF would be follow the mothership around, blast any surface deposits it identifies nearby, but its biggest job would be to follow up on any core deposits the mothership cracks. This would allow the mothership to instead move straight onto the next core asteroid, leaving the SLF to mop up the freshly exposed deposits.
Having our NPC crew be capable of operating mining fighters would also be the perfect opportunity to introduce ranks other than combat, as mining obviously shouldn't scale with their combat rank but instead probably their trade rank.