These regulators are actually rising rate FMU's (fuel management units) they increase Fp when boost is applied....the last thing you want is to adversly affect that correlation between boost and Fp. While I cannot personally illustrate 100% cause and effect problems, it is cheap insurance to just avoid the whole situation by not T'ing off the regulator. There are tons of other vacuum ports and nipples on these engines...so, why risk it.
THis is only theory as far as I know, but so is gravity, and I don't intend to drive my car off any cliffs to prove Newtons theory, just as I wont be T'ing off the regulator and risking Pistons/ring lands/rod bearings.