The AG came about first in X-Wing, which was before TIE Fighter. Don't forget that as a Rebel pilot, you were making a part to NOT get in the line of any large Imperial operations for the most part. As such, those installations so often noted were nowhere to be found in-mission, because you were obviously out to, you know, NOT BE DETECTED. The only way the Empire can rapidly react to out-of-the-way guerrilla starfighter attacks is to have a starfighter which can jump in anywhere, any time. This is the gameplay-mechanic logic of the Assault Gunboat.
That said, the AG was never an opponent-balance element, as most of the TIEs presented enough of a challenge. The AG was there for issues when TIEs could not be. For this reason, it's still a valid, viable, and useful type.