The thing about having an aux is that more or less regardless of what the wind or sea state is, you are better off having one than not having one. Yes you might have 5kt of tide yes there may be 30kt of wind but even if all you can do is hold your position or run with the wind/tide, in most cases that's a hell of a lot better than being a dead ship going only where nature decides to take you, and in more normal situations of moderate wind/current it means a breakdown is a minor inconvenience rather than an emergency, and surely a dead ship is pretty much always an emergency unless it is shallow enough to get out and walk?
Apart from lack of physical space I can't see an argument for not having one unless you can afford twins, but that would have added a lot to the cost of my boat to the extent I wouldn't have bought it.
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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