So, yeah, looks like this thread has rambled off track a little, but it has been very entertaining, so thank you all! I love the OP's original question. I think you CAN overplan. But the judgement on that surely resides with the traveler who is doing the planning and their travel companions. I love to plan, not because I am a stickler for schedules, but because the months of planning before a trip are part of the joy of travel for me. I love diving into all the details -- ports, transportation, food and entertainment options, even rules and regulations. Learning about local culture, history, and potential adventures helps me stretch the fun of a trip for many months before I actually go. My spouse, on the other hand, wants me to do all the planning because he just likes to go with the flow. It is a match made in heaven, as I can plan to my heart's content, and when I float an idea past him, I know that he will always say, "that sounds like fun!" I make custom books for every trip with details about destinations, but I don't expect -- or even always want- to stick explicitly to the plan. Some of the most transcendent moments I have experienced in travel have been when we wandered off the planned itinerary because something looked more interesting.
There is no wrong way to travel, except to be disrespectful of the people, culture, or environment you are visiting.