##Why would this be the best experience? What is this magic 'experience' thing designers are always after? In our case, a good experience means being able to read stuff and understand the content. Typography helps us make that task as easy as possible for the users.
##But why would we, for an interface for an online encyclopedia and similar, need something so specific at all? As an Encyclopedia, reading stuff is the primary thing people will do our site. It makes sense to make that reading experience as pleasant as possible, because better typography will ultimately help people understand the content easier. There's more factors to good typography than the typeface, such as spacing, color, and size. These things need to be adjusted to
##If there is a very specific 'right font', why aren't we using it as a webfont?
I'm not saying we shouldn't use a webfont. I think webfonts are amazing, and as you rightly pointed out, they're a great way to make sure a specific font is displayed. 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 want to make sure that the user still gets a font that is suitable for, say, body copy. The default fallback isn't always a good choice.
To give an example, if I wasn't able to download the webfont on my windows machine, and no other fonts were specified, it would fall back to Times New, resulting in a pretty bad reading experience. Because Times New isn't very well readable at small sizes. I'd be much happier if I got something like Arial, which probably isn't as good as the fancy webfont, but still
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?
"As an Encyclopedia"? Is this about Wikipedia or MediaWiki, or maybe Encarta? Anyway, if it has some relation to MediaWiki please paste it on MediaWiki.org (here it doesn't even have a license, let alone being discoverable). A subpage like https://www.mediawiki.org/wiki/User:Awesomephant/Fonts (if you were registered with that username) would be good. Thanks.