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.