a) this doesn't include mobile/tablet browsers, thus it's irrelavent.
b) this is only the case because Google deliberately dropped support for a recognised format backed by open standards, in exchange for an arguably inferior format that it just happens to own.
Sure, desktop browsers aren't the trendy thing any more, and there's more PR value in having your website work on the iPad than market-share numbers would suggest, but for people not concerned with PR (say, hobbyists, rather than startups), this is a useful number to know.
In response to (b), also because Mozilla deliberately avoided support for a recognised format, in exchange for an arguably inferior format that Google happens to own. Which is to say, I'm not sure why that particular observation is relevant — however we came to be here, it's interesting to know what the current state of play is.
Then their stats are not realistic.
b) WebM is not a proprietary format