No buddy box is better than no instructor at all. Many have learned to fly while exchanging the transmitter with the instructor. Some instructors are reluctant to teach without a trainer cord, though, because the instructor isn't in control. A student can be reluctant to hand over the radio when he gets in trouble, and may crash in a situation the instructor could've saved. With a buddy cord, the instructor gets to decide when the student is in trouble.

You're quite correct that RC is different than full-scale. I've done both, and RC is more difficult, mostly because of the lack of perspective and kinesthetic feedback. (The consequences of a mistake aren't as great, however).