The system fonts are legible. With web fonts... sometimes they are and sometimes they aren't. And web fonts have that re-flow during page load, which is super annoying when you're reading the first paragraph and lose your place because the web font finally loads and it's a different width than the system font. If that's not an instance of illegibility, I don't know what is.
It's very simple to use font sizes and contrast ratios that are different to read with system fonts too, if you want to do that for some reason. Web fonts aren't unique there. The re-flow is a valid technical problem, and one that ought to be improved.
