Your disclaimer probably says it all. Why bother when there just aren't that many people on windows phone. It seems like a matter of "who is helping who?". if Google didn't support the iPhone, there would be an outrage and people would move to other video service because of the number of people on iphone. If google didn't support android... well that would be silly, wouldn't it? But there aren't enough people on windows phone for it to matter to google, however, there are enough people on youtube for it to matter for microsoft.

Except BB10 has a YouTube app.

Yes, an icon that opens up a link in the browser is a wonderful YouTube app Microsoft couldn't replicate: http://forums.crackberry.com/blackberry-10-apps-f274/why-the...

iPhone (and Android) have sufficiently-large userbases.

BB10 has an YouTube app, and that's just a fraction of the marketshare that WP has, so that statement is invalid.

BB10 has a compliant app built using public APIs that Google is suggesting Microsoft to use.