You know what the Halberd harasser reminds me of? The Gauss 'hog back in Halo 3. Same concept, pretty much: light vehicle, no driver-controlled weapons, and a high velocity anti-armor turret that requires a direct hit to kill infantry. The thing is, there are plenty of people with good enough aim to hit infantry with little/no trouble. So you've got an instagib AI weapon that can also fuck up tanks, mounted on a vehicle fast enough to outrun pretty much everything and durable enough to survive most of it.
What you're talking about is what one aspect the nerf is aiming to accomplish: forcing the AV/AI roles to actually matter.
In every multiplayer shooter with vehicles that I've played, the same thing has always been true: a light vehicle with a strong, turreted weapon will always outperform all but the very best armor and infantry players unless it's flimsy enough to die to one or two tank shells or sustained small arms fire. The three points of the vehicle combat triangle are firepower, mobility, and durability. The issue at hand is that the Harasser has all three in spades. The mobility is obviously the core; it needs to either be weak enough that it can't afford to loiter around or make what should be suicidal runs, or it shouldn't have the damage to actually kill much/anything by itself. Either a glass cannon or an annoyance, not a lightning bruiser.
Hell, I know there are problems with it, but there's one thing that Halo 3 did damned well: vehicle physics. When you hit a light vehicle with a shot from a tank, the fucker caught air and often flipped over. I'm honestly surprised, in retrospect, how much that game got right. There's certainly more of a rush for the driver when a single fuck-up means you're dead, rather than that you have to go hide for ten seconds while you repair. :x