When people zoom a page, it is typically because they want the text to be bigger. When we anchor the text to the viewport size, even with a (fractional) multiplier, we can take away their ability to do that. It can be as much a barrier as disabling zoom. If a user cannot get the text to 200% of the original size, you may also be looking at a WCAG 1.4.4 Resize text (AA) problem.
I already tend to avoid dynamic, viewport-width-based fluid typography techniques in favour of making just one font-size adjustment – at a desktop breakpoint – based on the typographic theory that suggests we adjust type size according to reading distance. I learned this in Richard Rutter’s excellent book Web Typography.
While the ideas and code behind the fluid typography approach are nice, Adrian’s discovery that it can hinder users who need to zoom text only strengthens my feeling that it’s not the best way to handle responsive type.