If it's important enough, I guess the registrar could be used as relay.

But I do see your point. On the one hand, I'd suggest an optional "reach the owner" part of whois, where one could list a physical or digital address (doesn't even have to be email, I sometimes list social network presence on "about" pages as contact method for unimportant sites, to prevent spam). But on the other hand, we already have a place where you can optionally put your contact details: the very website we're talking about. And there you can apply any anti-spam measures you want, which you cannot do if it's in a public directory somewhere.

I think it's fine if people have to publish their contact info themselves.