In a sense, curation can be the "secret sauce" in answering the broader user question (and I am talking about core stakeholders, here) "why should I care to put any time and effort into this site?"

Exactly right. While some people see curation rewards as unfair or unnecessary, others see it as a way to engage users and actually earn stake without having to be a content creator. And this is the most vital role that it can play - a means for onboarding readers and voters who have no desire to create content, but still want to actively participate on the platform and earn STEEM. It's also one of very few reasons for holding SP and may be the only reason for holding it right now.

No matter what we think about fairness, curation rewards have more than one valuable purpose for the platform overall.