The advantages of taking Cunard's, or any other cruise line's, excusions are that somebody else does the planning and takes care of all the logistics.
Cunard's price are somewhat similiar to other lines, maybe a tad high.
Alternatives for the ports:
In Genoa you can start your own walking tour right at the pier. Everything is walkable. Even the train station is right there in case you want to see another town.
For Rome many passengers take the train in their own for a fraction of the price of the transportation only excursion. Yet, if you have never been and look for a well organized overview tour, the guided excursions might be good options.