I'm thnking the same thing SD is thinking. It is easier to put the heel down forward, yet you still have to do a lot of compensating to get your body weight going again. It takes to much time to shift weight around.
I tend to do that, stepping forward with my left heel and push off as hard as I can with my thigh, straightening the knee hoping to compensate for lost power, agility and control.
The only time that I can see the heel being placed flat on the floor is during the break on the 4 or 8, if that... besides that the song would have to be way slow where the given style won't hinder timing.
The afro cuban class I took stressed never heel down, reason being that once your heel is down you loose balance, and it slows you down. They do, however, I don't recall the back foot heel ever been placed down completely. As it throws your weight way off... However, there are a lot of places, including the class I took where they would have the person go from toe to heel, heel to toe just to get them used to the feeling, but where never asked to do it dancing.
However, I have taken classes where a given style is to dance completely flat footed. The dancer must bend down a great deal to compensate for the flat footedness. I use it some times with slow songs, and when I want to look like I'm gliding on the floor.
If you notice SG, when you put your back foot heel down your weight shifts so much that it seems that you will loose balance, and you actually do. You will also have no power going off that leg, to gain that power you must first pick up the heel and get on the ball of your feet to then attack the next step.
Perhaps, she is really she trying to get you to do the hip action. By stressing the entire movement, like that you know how it feels and how all your body parts should be moving. Take in as much as you can, and as you mentioned, with time you'll have your own style. Use things as you wish, how you wish, even if the heel down limits you in some areas (If used when actually dancing) it can help the look and feel of something else. Incorporate it to your dancing, but IMHO, She is just trying to show you the body movement...
You are lighter, more agile, and more powerfull with your heels off the ground. There are moore downs, than ups, using a heel down style when dancing. There is just too much movement required and a loss of too much energy...