Pilums are still good. You cannot rely on them to damage things at a predictable time, but they are good at mauling ships that cannot defend themselves, like those who are pinned by overwhelming firepower from your Dominator or Onslaught or, if there is a large enough swarm of Pilums, any ship dumb enough to try to shield tank them all at once.
In earlier 0.6.5 releases, Pilums were faster, and with Missile Specialization, enough to replace Harpoons. It made enemy flagships with Combat 10 so dangerous that if you did not kill them immediately, it was a matter of when, not if, your ships would die.
I do not use Harpoons bigger than the 1 OP single missile version. They cost too much OP for too few shots that are unreliable, and they do not regenerate.