There is a difference between separating "web page rendering" from "browser UI" or "networking" and the kind of tab separation I am discussing: those are privilege separations, which at their bare minimum mean that when some JavaScript crashes, it doesn't take down the UI. This is both a functionality and a security benefit that I did not and will not argue against.
Chrome, however, also claims to isolate tabs from each other, so that one tab cannot affect the behavior of another tab; but, in practice, I have tons of tabs that all have ten totally unrelated websites rendering in them (everything from my e-mail client to 4chan), so that isn't actually offering me any advantage: it is still possible for rogue websites that are able to exploit only their rendering process to steal data from any other website that ended up in the same process.
Chrome, however, also claims to isolate tabs from each other, so that one tab cannot affect the behavior of another tab; but, in practice, I have tons of tabs that all have ten totally unrelated websites rendering in them (everything from my e-mail client to 4chan), so that isn't actually offering me any advantage: it is still possible for rogue websites that are able to exploit only their rendering process to steal data from any other website that ended up in the same process.