I still don't think it made much sense to recruit a NY cop into a NY mob family especially into a high profile position. Talk about waving a red flag in front of the NYPD bulls.
By the time Michael recruited Neri, the family had moved out of NYC to Long Beach. Vito and Michael were keeping a low profile vis-a-vis NYC activities. The novel makes two relevant points: After the shooting, Vito said he'd never go into the city again; and, after they moved to Long Beach, Sonny had the police and politicians in Long Beach in his pocket. So there'd be little or no reason for the NYC police to pursue Neri and his connection to Michael. And, if they'd tipped the Long Beach cops to Neri's presence, those cops would promptly have told Michael about the contact.
SPOILER:The first serious job Michael assigns Neri is to murder Moe Green--not in NYC, but in Hollywood.
END OF SPOILERMichael was primarily interested in Neri for his physical force, and for the loyalty he bought by springing Neri from a prison term. The fact that he'd been a cop was useful--in fact, it was a key element in Neri's murder of Barzini.
