While a panhard bar may not be necessary, after putting it on my L I could tell the difference the first time I took it around my favorite curvy road. And the amount of lateral movement isn't much, likely a half inch or less. Another nice thing is that on Lightnings, every one I've seen, the driver's side rear tire is tucked further under the fender than the passenger side. With a pan hard bar, I was able to adjust that so they are both equal. On Lightnings, the drive shaft is off center to the passenger side, as is the differential center. It's no big deal, I just never liked how far under the driver's side tire is compared to the other side. A little preload on the bar, and it's fixed. Now, if you think that's bad, on NASCAR vehicles, they adjust the bar while driving.
Oh, my 2007 Mustang has one from the factory. So hence, FORD must not care about the lateral movement.