If you pay Slate to read their articles, you get a version of Slate aimed at making readers happy.
If you read for free, you get a version aimed at making advertisers happy.
In both cases, Slate are concentrating on their core value proposition.
I've tried to not bombard readers with a terrible experience and they appreciate it. Advertisers stick around and renew regularly, signaling that the real estate is still valuable enough to pay for. So in my experience, you don't have to sacrifice user experience to collect dollars from advertisers.
Of course, there's no one size fits all solution for monetising a site - depending on your skill with ad sales (sounds like you're selling direct?), your demographic, and so on, different solutions will work better.
If I clipped enough content from that particular site, I'd pay them for single page viewing. Just so I can clip it.
From a marketing perspective it gives potential buyers a "reason" to pull out their credit card. As I understand it, Slate's ad-supported offering will continue to exist as-is, with pagination and a "view as single page" options. Logging in just makes single page viewing default.
The article gets at the tension publishers face. Rely exclusively on CPM ads and you get clickbait, but there doesn't seem to be winning approach on the paywall front. This seems like a really good way to test the process.
In a funny way, it's a great test of the value of UI. Is single page viewing by default worth $5 a month?