For what it's worth there is actually no plot at all in ACL or Company or Follies. While ACL and Follies take place at an event and so there's a beginning and end (the audition begins, and ends / the reunion begins, and ends), that's it, there's no plot or being driven by plot as a musical.
So absolutely, there is no reason to associate a good musical as needing a good plot.
Nine is a good musical and a concept musical BUT it does have a plot compared to these other examples.
