What makes the front wheel brake a bad idea? I almost exclusively use the front brake, because that's the one that'll actually stop me. (Source: when I used rim brakes I had to replace the front brake pad three times for every time I replaced the rear brake pad.)
I know some people use the rear wheel locking up as a way to sense when they have applied enough front brake, but that's not necessary. It's also possible to learn to sense that directly.
When going at a high speed, breaking hard with the front wheel alone can force the bike to rotate around it and the rider might enjoy a very short but very thrilling flight. Which I learned the hard way many years ago
I know some people use the rear wheel locking up as a way to sense when they have applied enough front brake, but that's not necessary. It's also possible to learn to sense that directly.