While I've never stayed on the ship the entire time we were in port, I have gotten off for only a short period of time.
I don't think it's weird at all and many people stay on the ship in certain ports. There is plenty to do, although not as much on port days. There are, however, often very good deals in the spa on port days.
My favorite days are days at sea and have been since my first cruise. Enjoy!
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04