I started flying on 737-300/500s which had already the AP/CWS buttons and the disengage bar. Using that one could leave it in a position where it was ever so slightly displaced and prevented any autopilot engagement without it being an obviously visible cause. Therefore it was drilled into us not to use the MCP disengagement bar. The resulting mode confusion and then the resulting confusion why the autopilot was not working was simply too distracting for many pilots.
The NG initially had a similar setup until it was replaced by the updated MCP with a disengagement button instead of the bar that had to be pushed downwards. Of course, using the CMD buttons a second was always possible, but not used as a standard method either.