I think a cruise in the Med in many ways is better than a land vacation. No question you don't have the time or ability to see as much in each port... but in many ways it's easier than a land vacation.
On a 12 or 14 day land vacation in Europe it would be very difficult and strenuous to get to as many different areas. The time necessary getting from the ports to the "cities of interest" would also be eaten up by the time necessary to pack/unpack and get from place to place.
And a BIG factor... Europe is much less expensive by cruise ship than it is by land, when you factor in hotels, meals, etc.
As AR suggested you view a Med cruise as an taste test, and note the areas you'd like to go back and spend more time in. One could easily spend 2 weeks to a month in Italy, and still not see everything.
There's also no question a cruise on Med itineraries is BUSY, and tiring. Days are long, and often you don't return to the ship before 8 or 9 PM.
If you're looking for relaxing, these itineraries are really not right.
The exception is something like the transatlantic cruise we're doing in Oct. Rome to Ft Lauderdale. We start off with a fairly relaxing 7 days in the Med, then 5 sea days coming to the Caribbean, a Carib port, and then another couple of sea days.