I would only run one if it is needed, and you won't know that until you drive it. It is better to size the calipers to achieve the correct bias than it is to use a prop valve. On a 4 disc system I would be much more inclined to use an LBS than I would to use a p-valve.
...I thought you said yourself, it doesn't adjust pressure? And parse the verbiage how you'd like, but by adjusting the so-called knee point in an adjustable P/V, it alters the rate at which rear brake pressure is applied in comparison to the front brake pressure. Again, it's not actually changing the pressure, but the rate at which it's applied based on either a fixed or progressive rate spring inside the p/v along with a shuttle valve.All that it adjusts is where in your pedal effort do you go past the knee point and start reducing rear brake pressure.
To me "off the shelf" kits are the demise of what hot rodding is really all about. I don't mind bought parts - I'm not that much of a curmudgeon, but a "bolt-on kit car" has nothing of value to me. You could probably tell that from looking at the Valiant's build thread...
We used an LBS on a 4WDB converted 4WD T-100 and I was very impressed by it. My Valiant came to me with a p-valve, so I used it (disc/drum), but long term I want to replace it with an LBS. Looks like BC Broncos sells the LBS for a little less than does the mfg that I linked above.