All it takes is for the user to make a mistake once, confuse the testnet with the mainnet coins and send his real coins to somewhere where he can't recover it. Terrible advice.
That’s why topics like this one are useful as they can
guieguide newbies and show them the best practices when using the testnet.
Besides, you should make your tests on the testnet then move to using the mainnet only after you figure out how everything (or the basics, at least) works. After that there will be no need for a regular user to use the testnet again. So, he can delete all his test wallets.
Testnet is for both developpers and regular users (including beginners) who want to test how bitcoin works without risking real money.
RegTest, on the other hand, is mainly for developers.