It takes very little time to work out when an interest free credit agreement ends so you get another card.
The only downside is you are locked out from the credit card company you cancel with for a minimum 6 months.
My view as well however I am sure by signing up to direct debit you are also locked in a contract.
All the reasons why I pay in full with an interest-free credit card so I can dictate the terms I pay it off and avoid a financial penalty.
Perhaps because by doing this you are committing to a contract for a season and by cancelling are breaking it.
Not much different from say cancelling a mobile phone contract early