Showing different things to search engines and users is deceptive. You're misrepresenting yourself to the search engine.
I want search engines to penalize sites that put content behind firewalls, because those sites are less relevant to me. If the site is good enough to overcome that penalty because the content is so good that it gets enough google juice anyway, awesome. But they should still be penalized for hiding stuff behind the paywall.
It's not a moral judgment on paywalling content, it's a practical one. As a search engine user, I don't want Google to show me paywall stuff unless it's really, really good. So Google is making the appropriate ranking by penalizing paywalled content. But if you trick Google by showing googlebot something that you don't show users, that's obviously deceptive. Which I will make a moral judgment on.