My recollection of the research/PWB split (I wasn't in either center) was that research (wisely) decided not to get into the business of software support or committing to a stable system, so PWB forked off to be a supported, stable version of UNIX. It was used by many of the computer centers within the Labs because of the promise of support and stability. Support included published manuals, with the hand of Ted Dollota (whose first language wasn't even English) creating some amazingly good documents. There was some idea exchange between research/and PWB (like the Mashey shell and the Bourne shell incorporating support for what is now thought of as "here documents"), but research was focused on innovation and PWB was focused on support. AT&T management no doubt were looking to an OS on their (IMHO mostly disastrous) entry into the computer business, for which they agreed to be broken up.