You've seen pro and cons so far. If we are unfamiliar with a location, we book the ship's shore excursions. We just don't want the hassles associated with an unfamiliar location, unfamiliar people, etc. And, as one mentioned, if the tour is late...well, that's not worth a few $ saved.
However, if we are returning to a port we've toured before and have some familiarity, them we might just ignore the tours altogether and just browse around on foot.