Normally private tour guides arrange your customized tour, you are the owner of your time, you decide what to do and when.So you don' t have to waste your time in places you don' t really want to stay, many of the cruises tours nowadays has a lot of shopping stops and maybe the guests preffers to spend more time discovering the history.
Of course depends of your port of call and the tour agents they has there.
In my personal opinion, specially cruise passangers have alimited time to discover the port of call, so sometimes is better to look for private arrangments.