I find the use of absolute values rather strange. If you feel you need a cushion to avoid obstacles by more than the PANS-OPS margins, add a cushion. But it really doesn't matter what height above the threshold that corresponds to.
I'd agree for a non-precision approach but following an ILS does get more and more tricky as you get closer to the threshold so I think the 500ft recommendation has some merit. I've no idea what the 600ft height is for though. Sounds like a committee meeting in which someone proposed the 500ft value and someone else said "and NPAs are less accurate so we should recommend a higher minimum for those"...
M.