Redundancies. On a self contained boat, redundancies are very important. When you are out at sea with no land in sight and your sails have been riped to shreds by a squall, you will be glad that you have an engine or extra sails. What if the engine wouldn't start? Without another system of propulsion you would be at the mercy of the ocean.
Redundant systems are good, but it is more important to have non-integrated multi methods of accomplishing a task. That way if one of the methods is destroyed or not functional, we will still have another. Examples of other systems that we have that have non-integrated multi methods are water, power, and navigation.
On the boat we need the redundant systems, because most of the time we will be somewhere were we will be the only repair shop for a 100 miles.