Our accounting firm does it, actually each firm I've worked for has done it. Unless they are a particularly dodgy firm, no they cannot take out more than they are authorised to take. As part of the regulations of the governing body my boss is part of (Chartered Accountants) we have to have our trust account audited every year by an external auditor so there is no way we would get away with taking a cent more than we were supposed to take.
A lot of accounting firms, especially small firms, have a lot of trouble getting people to pay their bills. We've had clients get thousands of dollars worth of refunds back then not pay our bill of $100 - $200 for 6-7 months which is so unfair. I guess that's why firms are now preferring to have the refunds go to trust accounts because then they have more chance of getting paid for their services.
We offer it but the client has to sign off and agree to it before we do it so they can still choose to have it go directly to them, we then just have to hope they do the right thing by us and pay us.