I definitely agree it doesn't make the front side weaker. It all happens simultaneously. Also, once the actual swing starts, the hips stop coming forward; so IMO he hips aren't "floating."
When I was younger, I worked out with one of the head hitting instructors for what was then the Montreal Expos. At the time, I started on my toe (much like Upton, only higher). His response to me was the farther I am up on my toe, the farther the heel has to go before I was able to start my swing. This was part of my problem of getting my hands through on pitches inside. He wanted me to try to get off my toe if possible (which took many repetitions because that is a habit). When I did, I was able to get to pitches much easier and wait longer before "starting" my swing.
I think that the heel is kind what starts our swing to the extent that it has to be on the ground before we can start. I think for Upton, he times his pitches so that once the heel is on the way down, he is deciding whether or not he is swinging. I would like to see a video of him taking a pitch.