This is an interesting topic for me because I hold a handful of domain names (10-20) that I'm not using and have often had this internal debate.

I have them because they're ideal names for a few side projects I've been considering building now or in the future, and I hold on to them because I want to make sure I have them if inspiration hits for any of the given projects.

I'm sure many of you own some "aspirational" domain names like this, and while I realize it's a much different scale, why is it functionally different than the OP? Is there a reasonable quantity of unused domain names for an individual to own before it becomes unethical?

I wouldn't want to hold a domain from someone else who wanted it in good faith and was going to use it (like I hope to, eventually), but if I want a good domain name for a product I'm building in the future, I have to play the game a bit ahead of time.