Those direct debits don't exist in Belgium. Maybe that's why giving out a bank account number doesn't feel 'unsafe' to me.
It seems to me those direct debits are more trouble than they are worth though. Why would you need those in the first place? Anyone who wants to make a legitimate deposit can surely be bothered to ok the transfer?
Direct debits are the mechanism in the UK that is used for paying monthly bills automatically. You set up a direct debit instruction with a company and they use it to collect the money needed to pay your credit card bill, phone bill, magazine subscription etc
What it needed is a separate number that can be used to pay money into your account, but can't be used to setup direct debits.