I don't have any science here to back it up but I think some guys will still get nip flare ups while taking a Nor-19 with an AI. A SERM seems to help limit this better than an AI. I think that the estrogen conversion from a Nor-19 also revs up prolactin which a SERM can help and an AI cannot.
(I'm mostly thinking to use the SERM to prevent nip/gyno issues- really nothing else so if I guy is not prone then an AI is probably ok.). As always trial, error, experience, and bloodwork will be different for everyone.