FYI-the best way to never have to deal with the injectors is to supply good clean fuel to them. That involves where you buy your fuel and how you filter it on board.
Carrying the spare injector is a good idea, it saves ordering one when time may be a factor. You're not going to be changing these out at sea unless something is really, really wrong. The norm, when the engine has a miss, is to pull the injectors and take them to a mechanic. He has a hp pump that he attaches to each one and can observe spray pattern. The same pump is capable of pumping cleaning solvent through the injector. If he cannot clean the injector up adequately to provide an atomized spray pattern, it must be replaced. The problem with bad injectors is that, instead of atomizing fuel, they spray liquid fuel, usually at one angle, right on to the cylinder wall effectively washing away all of the cylinders lubrication. The damage done to the engine is not so much from mis-firing as the degradation of lubrication on the cylinder wall.
Twenty-five years ago engine builders were well on their way toward standardizing injectors across engines and injectors were $20-30 a piece. Today, every engine has it's own specific injector, and $100 a piece is considered reasonable. Go figure.