My last cruise was a Celebrity out of Barcelona. It was a 10 day cruise stopping in 4 countries. I flew to barcelona 14 days ahead of my husband and explored Spain on my own. I mainly stayed in cheap budget hotels and hostels and took local buses and trains. I even went to Lisbon for 2 days and had a blast.
After vagabonding around Spain it was nice to board Celebrity Century where everything is planned and you are treated like gold.
However if I had only been restricted to the Cruise and had not been on my own experiencing Spain on my own terms I would have been absolutley left wanting more. In other words a cruise is just not enough (for some of us).
I think combing the two is best for me.