I'd be okay with training students with very little "air-only" practice. I wouldn't be okay with training students with very little "more than air" practice. I've seen people who trained almost exclusively with air and makiwara (the hand-held ones, which cannot be struck hard without potentially injuring the holder). Many of the students didn't really seem to understand how to deliver power and impact. They struck from too far away in application, because they weren't accustomed to targeting with distance for realistic power.
I prefer a good balance of the two. Moderated practice without resistance is useful for examining form, checking that the student is holding balance (rather than expecting the target to rebalance them), and to prepare for when you miss. Most of the practice should be done with some sort of resistance: the clapper, makiwara, focus mitts, padded shield, heavy bag, BOB, or a human partner.