Yeah I think the field defenses should stay. Riflemen mines are more useful combo with the Pershing in the long run imo. Having a mine on your mainline makes it easier to keep planting them in the late game
Not to mention you can always just get zooks on REs to give them other usefulness
I like the field defenses more too, it was just an idea to make echelon builds more viable if the Pershing needs them. Technically, it would also slightly improve the ability distribution, since 3 commanders have field defenses but only one the assault kit.
One issue the Pershing has is that it forces you into a very rigid build.
You need an additional Jackson for AT, probably a Sherman for some more flexibility. Together with the ambo you're already at 49 POP (which is a good value for inf/vehicle balance). But since you also need 3 Rifles, one officer, the major and at least one echolon unit, you're up to 86 POP, but only have 4 (/5 with the RE) squads to effectively cap.
I had the feeling that at least Ostheer Tiger builds could slightly save on POP for another squad of flexibility, since the Tiger has so good AT that support by a pop-wise cheaper PaK or StuG is sufficient. 4xGren, Tiger, StuG, P4, 2x Pio build costs 81 POP. With the necessary squad weapons you also go up to high 80 POP, but you have usually 5-7 squads that can cap. Optionally, you have the choice to replace the StuG/P4 with other units (PaK/Panther/Brumm/PWerfer) depending on what you need. As USF, the build described above is pretty much it, you have 13 POP to experiment and react to your opponent. So basically the Ranger squad, unless you throw a Rifle to free more POP. It's not necessarily a bad build, but you don't have many options to react.