Ick. I hate doing two-finger taps as it is. I don't believe one should need anything other than a pointing device with one button to browse the web effectively. Needing to hack in ways to simulate extra buttons is bad design. Why not use a keyboard then?
Even if Chrome always opened external links in a new tab with a left click, you'd still want middleclick functionality to browse the web effectively; I constantly open multiple tabs from the same domain- for instance, to open a HN comments section without navigating away from the front page.