I think I should have been more specific with my reply.
We never book any ship's tours until we are on board (and we often don't book any then either). But we sometimes do book the services of independent tour companies (like Red October in Russia), or taxi/limo services in advance. We also research what sort of services (local public transportation, intercity trains, hop-on-hop off busses, etc.) may be available at various ports and make our plans accordingly.
Of course we understand that in the Caribbean you're often stuck with ship's tours because there's very little else available. We're averse to tour busses and try to avoid them (and the cruise lines' ever-expanding markups) whenever possible.
There's a very interesting article in Friday's Wall Street Journal if you can get hold of it, or check it out online. It's about the trend toward longer port stops and more elaborate shore excursions. It has some fascinating stats in it. The print title is "New in Cruises? Less Cruising."