I can't see how they could provide any value above anyone else in the market. The moment one site is willing to fund themselves purely on adverts is the moment that subs based sites all start to crumble.

As a developer I use sites like SM every so often when it's quicker for me to look something up than to figure it out myself (when time or cost are more important than my pride). I'd never ever consider paying directly for that info (time and cost then swing back under pride) unless someone smarter than me was available to live answer questions, which will never happen.