I think it was Cameron Hewitt's talk where he discussed various ports and possible excursion options. He discussed which excursions make sense to do with the cruise, some that may or may not, and some that you absolutely should do on your own. Depending on your cruise and cruise company, it can possibly make sense to do their excursions.
I carefully went through all my excursion options and a very generous, well-traveled, forum member was kind enough to sort through them with me. We are going through Viking so there are a lot of free excursions. Of the optional excursions, I did find a few that I liked that would be hard to replicate. One excursion is fairly easy to replicate (Athens Acropolis), but by the time I added up transportation, tour guide, and tickets, I was at the excursion price, and the cruise excursion does make it all easier. To finalize this decision, I did look at reviews of the Viking Acropolis excursion.
Another example of my thought process, my cruise had excursions to walk the walls of Dubrovnik and Kotor. When I priced those out and keeping in mind the ports are in/close to the old town, the excursion price was ridiculous and it will be super easy for us to walk the walls on our own.
In regards to Pompeii, It is easy to take the train to Pompeii from Naples. However, the other posters make good points about using a private driver to maximize your time. You may want to see if other passengers want to join you to reduce costs. Many do like a tour guide for Pompeii. We did not find it necessary. We did some research in advance and then used the RS guide to aid us in exploring the area. That said, we were there for the entire day. You may want to get a tour to do it quickly/efficiently so you can go to the museum in Naples or spend some time exploring Naples and/or eating pizza.
Lastly, in regards to your "reluctant" cruise comment. I wouldn't feel bad about trying a cruise. My husband and I have done a lot of independent travel in North America, Europe and lately, New Zealand. There is nothing wrong with trying out something like a RS tour or a cruise. For us, at this point in time, a cruise met our wants and needs, and our ability to travel independently does allow us to go off and do things on our own when it makes sense.