TPCs are adequate, if not optimal, at anti-armor when every other significant gun that points directly ahead is a needler. I use TPCs as anti-armor when I cannot rely on Devastators (or missiles) to break armor.
There is one other (occasionally) useful role Onslaught is good at: troop carrier. It (and Legion) can haul significantly more extra personnel than other capitals, and most other ships for that matter. The last time I brought thousands of marines to factions' capital worlds to steal items from Sindria, I did not bother to bring Valkyries or Colossus 3 (because they do not bring enough marine power, despite Ground Support Package), and I did not want to build or bring Starliners. Instead, I stuffed my fleet with many Onslaughts like 250k+ named bounties do and spammed them in fights. Onslaught can brawl and bring lots of marines to raid planets.