I did a little testrun with 2 large unmodded Frags on my Chief and that didn't work that brilliantly. Went to a High RES, and apart from a couple of small ships that I was able to jump, it was rather a drag. I'm generally not afraid of the FAS or its siblings, but here I was fighting one forever and he kept getting his shields up at least 4 times; I just didn't manage to get close enough to frag him, we just kept circling each other at ~650m. Another big target (a Clipper or so) simply jumped away when I almost had him and there was nothing I could do about it.
Both of these situations are not such a problem with Multis; in a circling fight I can keep blasting away and score more hits due to better hardpoint placement, and when someone tries to run away the Multis can also reach significantly further.
I have a feeling that these issues might be influenced by my control setup. I only have flightstick and keyboard, and adjusting thrust with keyboard only is a very clumsy affair. Theoretically I know what I'd need to do to get close to a circler like the FAS, I just can't get the ship to do it with the keyboard. So maybe I do have to get a second stick if I want to use frags (I've been putting off that decision).
So for the time being, I'm guess sticking with MCs. Might do some test runs with PAs though, just for kicks.