The reason "the mids and upper bass from the main speakers are improved when you add a subwoofer" is because relieving the main speakers of reproducing very low bass (assuming you are highpass-filtering the signal sent to those speakers and their power amp) can greatly reduce the distortion they are producing. The lower the frequency a speaker is trying to reproduce, the harder it has to work (move air), and the more distortion is creates, generally speaking. If the woofer isn’t having to produce low bass, it can produce upper bass and mids freer of distortion.
That’s why many Maggie owners use subs with their speakers---not just to provide the very low bass that Maggies don’t, but to reduce the amount of upper bass and midrange distortion that results from relieving the panels’ magnetic-planar bass driver of moving far enough to reproduce frequencies below 40Hz or so. The same is true of many other speakers as well.
If one doesn’t highpass the signal going to the main speakers and their power amp, that advantage of bi-amping is lost.