Hey Isarra, that's some good questions.
I'll try to answer as concise as possible, but in case you're interested, here's the detailed version: https://gist.github.com/awesomephant/9352699

> Why would this be the best experience?
In our case, a good experience means being able to *read stuff* and understand the content as easy as possible. Therefore, a typographic setup that makes reading as easy as possible makes for a good experience.

> But why would we, for an interface for an online encyclopedia and similar, need something so specific at all?
Our goal is to help people get information they need by reading articles. It makes sense to make reading an article as easy as possible, because ultimately that will help people understand the content. The typeface is an important part of good typography and should be chosen carefully, even though there's other factors such as spacing and size that need to be considered.

> If there is a very specific 'right font', why aren't we using it as a webfont?
I think webfonts are amazing, and we should definitely use them. However, even with webfonts using a font stack is a good idea. What if the user has an old browser that doesn't support webfonts? What if the user chose not to download font files to save bandwidth? In those cases we still want to do our best to ensure a decent reading experience, which isn't always possible with the default fallbacks.
Our font stack would look something like this:
'Fancy pants Webfont Pro', DejaVu Sans, Arial, sans-serif;

> Why did the generic 'serif' and 'sans-serif' become insufficient?
They were in fact never sufficient. But for quite some time, web technology didn't allow us to do it better. Now that it does (with webfonts and finer typographic control), why shouldn't we go ahead and improve our user experience?

Hope that answered your questions, feel free to hit me up if something isn't clear.

Best, max.
@awesomephant