When are those more useful though in my situation, though? I've already got the best anti-horde in the game in the form of wyverns, I can run a squad of 3 for 195. As for non-horde armies, the frags would be better than the grav against a poorly spaced 10 man
tac squad, but most people don't run full squads and they have to be in template range, where the gravs they just have to be somewhere within 24", maintaining their usefulness after the initial drop. The only instance that I think frag templates are better than grav is a non-invisible ravenwing deathstar, but that's a very specific case that I'm currently not building for.
In general I'm just really not sold on frag cannons, especially paired with a guard army. If I'm podding something in it's going to be 100% a fire magnet, guaranteed dead the next turn, as nothing else in my army can make an immediate threat for the first 2 turns. To make the unit worth the investment they need to take out something vitally important, and again, outside of a supremely expensive jinking unit frag cannons just won't cut it, while grav cannons can take out any vehicle/
MC/GMC as well as most expensive units in the game in one go. And even then they only need to be within 24", not in their face, so they might survive into a second round to shoot again, where you need to fully invest in a vet squad to keep your frag cannons alive (making it a 450 point unit).