Not to mention peace of mind - the ship won't leave until all the tours are back. If you go on your own you may have to build in extra time to be sure you won't miss the ship. If you're highly independent we're sure you can always do things on your own. But for the rest of us - the tours are just fine. And if you're worried about waiting for stragglers, this rarely happens in our experience, especially if you are on a cruise in Europe, Asia, etc.
Experienced cruisers know how to behave.
On a cruise, you do what you want to. You can do everything. Or lie back and do absolutely nothing. It's your vacation. Most cruise lines make a special point of providing supervised activities for youngsters, especially during school holidays.