It's "necessary" from an axle location point of view once you find that your axle is not centered closely enough to suit you for either tire fitment or visual reasons.
The main benefit to handling is with respect to "feel". This is mainly a function of aftermarker PHBs being supplied with stiffer bushings in either or both ends or some sort of spherical pivot in at least one end rather than from the bar itself.
I'm not exactly sure why the recommendation for a steel PHB for FI applications, unless the assumptions are heavy dragstrip duty, no LCA axle-side relocation and poly bushings in both ends of the PHB. But I'll take a stab at it.
There is a bit of pinion angle change due to rear squat under acceleration, which then twists the axle end of the PHB. Still looking in side view, the squat itself amounts to the chassis twisting the other end of the PHB the other way, adding to the amount of torsional shear stress that the PHB has to carry. OE rubber bushings are soft enough to avoid much torsion from developing in the PHB (and making just one end a spherical joint will all but eliminate it) but plain poly will cause a lot more to be developed.
This situation is a repeated load on the PHB, and aluminum does not have a definite endurance limit, so I'm guessing that the mfr is being properly cautious here.
Norm
