Because often things don't break right away. They just wear out prematurely or repeated abuse eventually gets them. If you have multiple people with varying use cases odds are someone is abusing the boat because they are trying to shoehorn it into a use case it really isn't well suited for.

If you are trying to bounce off 6ft surf (emphasis on "off" rather than "through" in a tri-hull) that will eventually come back to bite you in the form of tearing the vessel apart at the seams. If you are constantly beaching something with a deep V you can only go so long between events where the prop/lower unit unexpectedly finds the ground (and eventually that will get expensive). So you either share the cost of these repairs with the understanding that that's just the cost of doing business (because simply based on your varying use somebody will have to be more abuse to the vessel) or you all get your own boats that fit your use cases.