The problem isn't that the PAC/8 is useless. It's not even that the designer is cutting the MechWarrior's throat by employing one - whatever its faults, the PAC/8 is a lot less likely to actively contribute to the machine's demise than, say, the Po HVAC's main gun, and the Mongrel's ERMLs give it a passable throw-weight that the autocannon is just there to supplement.
The problem is that the PAC/8 really manages to coast into that spot not because it's a good or even decent weapon, but because someone was already silly enough to use the AC/5 of all things on the original and the PAC/8 fits the profile of "single small ballistic weapon with a bit of punch to it". Once you get outside that very, very narrow niche, the PAC/8 is up against Clan missiles and lasers as direct competitors. #P