Not all graffiti artists practice on buildings. Plywood is cheap.

Of course, tagging in public, perhaps in a hard to reach place, is a separate skill requiring its own practice... I'd just prefer they did it after having practiced the tag in their backyard first. Like Banksy.

Part of the skill in getting up, is getting up without getting caught. If you support full pieces, you shouldn't complain too much about people practicing with tags (although most people do complain).

The solution is of course to have legal walls. But that would be far too progressive for most councils.

Getting up doesn't require tagging.

Tagging doesn't require permanent paint.

Legal walls don't satisfy the goal of getting somewhere challenging.