If your graphic design is optimized for impact at a distance without scrolling, your use case is…

If your graphic design is optimized for impact at a distance without scrolling, your use case is advertising (or something like it).

The typical use case for the web should not be an ad.

Maximizing the amount of legible text on the screen encourages people to create informationally dense content — it means that a short article like yours looks lonely and empty, while longer articles seem like a more natural fit. This seems like a pretty good thing to encourage; I certainly prefer long, in-depth writing to shorter, shallower stuff.

To the extent that designers should be allowed to control default text size, for primarily written content, ten pixels is probably the upper limit for what I’d consider acceptable; any more artificially inflates the perceived size of a piece of writing. However, web designers have a lot more control over the way that sites look and act than they probably should for accessibility reasons: if I set my browser’s default text size to six, websites should accept that, the same way they should if someone with poor vision sets their default text size to thirty.

An increased default size for body text points to a set of values that are already far too common on the web: a preference for flashy showmanship over well-delivered content. Good design steps aside and becomes invisible; the exhibitionist design-masturbation of huge body text never can.