At least a morning star. Of course having a blob turret would be really bad. But, it'd be better than nothing. Assuming that blob turrets are a lot cheaper than the advanced lasers and plasma that fighters use. And i believe the subach is going to be the next generation blob turret, they're cheap to produce, more than plentiful, and the smaller ships besides fighters are sporting them.
But, a blob turret would be better than nothing to have on the back of a bomber since bomber designs are phasing out turrets period. Instead of phasing them out completely i'd at least try to keep a turret as long as possible no matter how crappy it would become before phasing it out. I mean all of the bombers with turrets have kaysers, a step down from that prometheus, maxim, etc. whatever. Or hell, if you want a turret that's as ****ty as the blob turrets but actually shoots fast and shakes up a target in the least i'd want a morning star turret. A morning star turret would still keep the enemies off your back really good too.