Thinking about this with a present-day analogue: fighters are like the aircraft on an aircraft carrier, while frigates are small sub-destroyer support ships, such as this:
http://en.wikipedia.org/wiki/Type_23_frigateYes, a carrier can resupply a fighter jet in perhaps half an hour or so (restock ammo drums and attach underwing missiles), and it is plausible that there are additional "fresh" planes that haven't been damaged. But restocking and repairing a warship? That takes
days or weeks, respectively, even with the proper facilities (a dry dock).
And besides, if you have frigates that can be repaired and restocked, what would be the point of expensive, high-FP fighters like the Xyphos? Yeah, there are two Xyphos, but I can't think of many situations where I'd take a wing of Xyphos over a repairable Tempest.
Also, the Astral is hardly a useless ship. The number of simultaneous fighter wings it can effectively support is tremendous, especially if you stagger it. You can continuously bombard enemy crafts with a series of fresh fighters, supplemented by Hurricane MIRVs.