I tend to trend towards the "rebuild engine in mid-cruise" supply level, but I did decide the extra starter weighed a ton and took it home.
If you race, then you need two screwdrivers and vicegrips, but not the big vicegrips
* I impressed my brother when the alternator died mid cruise. In about 20 or 30 minutes we were back underway with the spare alternator. He thought we were going to be on solar power the rest of the trip.