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You're right, with 3 heavy fighters you can't take down an Onslaught, but you can catch 3 points and deploy the rest of your fleet while the enemy slowly moves his battleship towards you.
Fleet cost alone is not indicative of firepower capabilities: heavy fighters are good as point cappers, can hunt down frigates and provide a priceless help in taking down interceptors and destroyers. But alone against a large opponent they are dead, it's their main weakness.
Also, remember that there are several "categories" of fighter crafts:
- interceptors: they move quickly through the battlefield to catch small ships and bombers before they can do damage.
- heavy fighters: they can devastate interceptors with superior armament and endurance and can damage frigates severely, but they're slower and their armament is not designed to harm anything more than a destroyer.
- assault fighters: these guys bear a lot of guns and armor to devastate enemy ships, but are vulnerable to faster and smaller crafts.
- support fighters: they support (duh) ships and bombers with armament designed to destroy missiles and help bring down enemy shields.
- bombers: they drop bombs (double-duh) and torpedoes on enemy destroyers and above. Devastating, but usually with very little armament for self defense and slow speed.
If fighters were cheaper (FP wise) they'd be too powerful, right now they have their role in the battlefield, just not the protagonist role.