Similarly, we try to support tablet users (iOS/Android) with external keyboards or mice. I totally agree with your point (I purchased a laptop with a touchscreen for testing).

For tablets we optimise for touch, but I also make sure keyboard and mouse work (e.g. I am extremely careful to support click events, not just touch events). I can do this because we have our own framework (when I developed our product, other frameworks simply did not work well with corner cases). Writing our own framework means I am very familiar with the various quirks and workarounds.

However, browsers force us to make compromises, because we simply can't detect or fix certain problems that affect the UI.