I'm aware that depth charges are part of FAF because the Czar drops them.
Ironically, the weapons of torpedo planes are also classified as depth charges (perhaps they are only classified that way so torpedo defense doesn't nullify torpy boys).
I think it would be fun if it mattered whether frigates ran over subs. It would turn the tables a bit on subs harassing frigates (subs would have to be a bit more cagey about it but of course subs, properly microed, should be able to beat frigates).
Probably the depth charge weapon of frigates should not be available against buildings or floaty units (hover, surface ships, or walking on the surface like engineers), it would only be against submerged units. There's no reason to give frigates extra anti-building dps. It wouldn't make 1-2 frigates very dangerous for subs, but a big pack of frigates could kill subs if they spread out and the subs couldn't escape. Which would promote the sense that subs are "hunted" and need to be sneaky/snipers even when they're in a position to do damage.