The main reason that the mobile menu switches where it does is to conform to the switch points of sidebars. I believe it would be quite disconcerting to have the menu switch at a different point than the sidebars switch.
I do see that having a “don’t use mobile menu” option might be useful for small menu sites, and I’ll think about that one.
But as for @nilolaus – do think a bit about what happens 99% of the time when people visit you site. I don’t have hard numbers, but very few users actually resize a desktop browser while visiting one page. So in the real world, only a tiny number of people will even see that switch when they view your site. Just a though.