If you surf without a leash you would learn slower. You would not try to do stuff where you might fail, always surf within safe limits, etc. For a beginner IMHO the leash keeps him closer to the board giving him more time to actually surf and learn faster. No matter how well you swim, you’re not gonna swim well through a surfzone. I’ve seen surfers go out and pick up swimmers that were only 10 yards from the beach in Biaritz that couldn’t get to shore due to the strong rips.
I think the idiots that are not safe in the water would still be reckless without a leash.
regards,
Håvard