the only thing they seem to be good for is buying time till my fleet arrives
Which is precisely what they're supposed to be good for.
I wouldn't argue against improvements but as has been said a million times around this forum, they aren't supposed to be single-handedly stopping fleets. They exist to buy you time.
Plus, unless we're talking about LRM frigates that were staying out of range and not getting shot at all, 10 frigates shouldn't have a chance against 15 turrets.