The advice you quote is all in the spirit of keeping things lean and mean and clean, particularly in the case of a server. I'm not aware of any strange behaviour. In most of my testing, the PXE rep is the core. It's OK for testing.
I think there used to be some flakyness, I know, I saw it and experienced it myself...but no one ever really could figure it out.
Then a few releases ago, the training departement had the install team make sure the PXE rep could install on the Core for their training labs (cause they usually only had one Core and one Client so for testing the PXE Rep had to be on the Core.
After that I never saw the flakyness again.
It's really just best practice so that you are avoiding unnecessary load on your core as nowadays with WinPE there is a lot of activity associated with a PXE rep and having lots of systems continually taking gigabytes of data from that server is probably not conducive to a highly performing core.
That said, I always have my Core acting as the PXE rep because it's easier for my test environments. If this is going to be a production server for lots of imaging, probably best to split it off; if not, then put it there for occassional use and an easier infrastructure.
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