I'll step back in to make a counter-point here, Headdie: A tactical fighter which can move independently of a large strike vessel is still immensely useful for obvious reasons, and any competent military force would field such a type. As you noted, there aren't massive numbers of Gunboats out there, and they exist as a specialized type - a fighter which can perform roles no other Imperial type can. Due to the large number of Imperial installations out there, as well as the comparatively small number of Gunboats, you'll not commonly encounter the type. But, if you need a fighter to act as a long-range escort, a tactical interdictor, or even an atmospheric close support fighter, etc., you genuinely need the AG.
The only other "common" Imperial jump fighters only become common after the EU really starts to set in, and these are most notably space superiority types. The AG has a definite, useful, albeit uncommon purpose. If you need to slam a target with big shuttles, the Lambda and the Landing Craft are there, and the DX-9 has ion cannons and torpedoes. The Gunboat is the bread and butter in between your heavy boarding and assault ships, and the only one that can escort them (barring your advanced TIEs which only come about much later) after they jump.