6 seconds between shots is not an issue? The burst of damage (1250) is huge overkill on most fighters so you're actually not doing anywhere near full DPS if you don't hit more than one fighter, especially if you're facing the more numerous wings of fighters (Retry is spot on IMO). The biggest problem scenario for phase lance is sparks that appear in the hardest fights in the game in wings of 5.
The player can mitigate some of this by lining up fighters to kill more than one per shot sometimes (on maneuverable ships that can kite), but it's still definitely a downside in AI hands. You're also definitely not hitting more than one fighter per shot consistently unless you manually target which is impractical for non-overlapping rear-facing mounts on the apogee. I use phase lance primarily as an anti armor/hull weapon that can be useful for shooting down bigger fighters occasionally. I don't think it's particularly close to good enough at anti-fighter to be used purely for auto-fire anti-fighter in rear mounts. That's my personal opinion, and you're free to disagree, but there's at least logic behind it.