Those keywords can only be used in the shorthand ‘font:’ property and only if they’re the first value (to try to avoid being mistaken for a font-family of the same name). They set a lot more than just font-family, not just other font- properties like font-size and font-weight but also line-height. There also seems to be some variation in what is set by different browsers on the same platform; when using the ‘small-caption’ keyword, Firefox for Mac (53) sets font-weight: 700; while Safari (10.1) and Chrome (58) do not.
Those all seem like obstacles to using those keywords as implemented today. However, I agree that working together to make improvements through consensus, or at least with consultation, is better than going it alone.