Not to completely take this thread off track, but I always wondered why this is? Since I got my spurs I've had to relearn how to use my lower leg... I used to point my toes out, too, but that doesn't go over too well with the spurs. D: I was just wondering why you don't want to use the back of your leg if you don't have spurs.
Not a problem, ask away :)
Just sit there on your chair - computer chair, couch, whatever - on the edge. Put yourself into your riding position.
Start with toes forward where your inner calf would be touching your horses side. Feel where your hips are positioned, feel where your knees are positioned, feel your 3 points of your seat *both seat bones and crotch*
Now, turn your toes where they are outwards where the back of your calf would be on your horses side.
Now feel what happens to your hip area, your knees and your seat bones.
It is about effectiveness of your aids, to beable to bring out the best in your horse. Your solidity and effectiveness is altered when you are riding on the back of your calves.
You want your lower leg to be solid at your girth and around your horses side. This improves your effectiveness in your tack, your solidity, your form.
You don't want to strong/nagging of a leg, nor do you want to soft, quiet of a leg - you need perfect balance. An effective leg when needed, a quiet leg when needed.
See how effective a riders form is, when their calves/legs are placed correctly on their horses side:
This rider here shows an excellant example of the proper angle of your toes/calves on horses side:
Does that help?