It really depends on the relevant counterfactual; yours makes total sense from the vantage point of lots of developers, but I tend to prioritize privacy and autonomy. When I visit catphotos.wordpress.com, my intention is not to leak information to Google even though they have great fonts. My intention is just to visit the website.

So the counterfactual I would frame the discussion with would be something more like self-hosting fonts by default and prioritizing privacy over performance (different strokes for different folks, and I realize it can be a significant performance hit).

To respond to your "more common the hosting site is" comment, Wordpress is also extremely common, and they probably could have devised alternative solutions by making different trade-offs.

Cache behavior resulting in fewer requests can be a double-edged sword, too: if you cache fonts with clients, you're probably also caching a bunch of other things that may decrease your privacy in other ways. There are many layers of indirection, especially with NSA/GCHQ/Verizon.

I wouldn't argue that this and another services offered by Google don't add value for developers and even users (they absolutely do), but my argument is mainly that there are costs--maybe distant/abstract/indirect costs in terms of privacy/autonomy that are difficult to discuss in concrete terms, but costs worth considering nonetheless.

I wish WordPress had been more thoughtful about the trade-offs they made.