Speaking for 2 people who just did private tours in 2 ports (Livorno and Civitavecchia)...it IS expensive for just 2. We spent, between the tours and the tips (cause yes, I'm an American an overtip) around $1100-1200 (depending on the Euro, I was able to prepay one in Dec; can't remember what the rate was so estimating). That's about $300/per person/per port.
The pros for doing 2 people tours: 1)total control. I'd had a migraine the first two days of the trip so it was a relief to tell the drivers-"we're done, take us home." 2)Riding in a nice sedan v. a bus. 3) Less walking in Rome...our driver knew Rome's twisty turny roads and alleys very well so he could get us very close (say 1/2 block) from where we wanted to be, park ?illegally and be waiting for us when we were done.
The cons: 1) The expense (see above). 2) No other people...since I'd been ill we really didn't interact with anyone else (missed dinner every night) and if you're a people person...well...3) Someone still has to research and contact the company and make arrangements and decisions. I didn't want to go to Florence in the summer (crowds!) so we went to Cinque Terre and that was sort of a disappointment. So a day in port kind of wasted.
That said, ABDs are still more expensive than private tours (looks like for a 7 day cruise there are 4 port days and it's a little more than $2400/pp so $600/per person/per port. Pros as above-more excursions,
ABD guides, snacks, lunches, and as you said a sort of preview of the ABD concept. Cons-$$, still traveling with a group, buses, no control over what you do. Noting that you don't expect to return to the area anytime soon, it might be a definite advantage to doing as much as you can.
Good luck making your decision and have a great cruise