Direct debits mean that the company / service provider has control over when they get paid - not you.
I've worked in financial services for 14 years and have seen the hassle and misery direct debits can cause when they are processed incorrectly by the originator.
As a result I pay all my bills by my own means - internet banking, post a cheque etc.
Yes, the Direct Debit Indemnity scheme means that people will be re-imbursed - but not instantly. Frequently people are left with their account in a mess / without funds because, for example, an insurance company has decided to take out the full year's premium instead of monthly instalments.
Or a gym [probably the worst offenders] takes out two years' worth of payments in one go - some 18 months after the arrangement had been supposedly cancelled.
This happened to a friend of mine on the same day of his father's funeral.
As a result he was unable to pay a local hotel the bill for the mourners' soup and sandwiches as his laser card was declined.
Hassle he could have well done without.
Direct debits: avoid.