"Should we all just use (say) chromium"
No, no, no! And it's not to do with the "competition" angle mentioned, but as soon as you assume a browser quriks, you end up locking out systems that don't even have that browser available.
It would be a return to that "best viewed in internet explorer" or the ever inappropriate one often seen on commerce sites "This site will only work with internet explorer 6. You must upgrade to continue" - A lovely way to treat potential customers who were most likely "upgraded" far beyond what IE6 offered.
In a similar vein, how many of these "browser quirks" today are real, and not down to badly written html/css/code that doesn't conform to standards?