We're working on it! We heard this feedback a few times. Our intention is to make mobile the default in DevTools, but we don't want to make the experience worse for people building Desktop sites. So we'll come up with a better solution. Stay tuned!

How about persisting the last state? So if a user switched to desktop, keep it as desktop when they re-open devtools.

The mobile device bar is particularly annoying while working split screen on laptop; vertical space is already very tight.

As it is right now, you can choose whether you want it split vertically, horizontally, or undocked with the dock button if you click and hold. This is just a different set of defaults if I understand correctly. It should be trivial to set it to how you want (and currently it remembers your prior preference, so hopefully that continues).

if you move the split from the bottom to the right side, it fits great... Unless I'm missing something?

I typically have sublime on the other side of the screen and the dev tools docked to the bottom - http://i.imgur.com/JByAoiK.jpg