You can actually get emergent conflicts from this in somewhat unexpected ways. When users are enabled to export a bunch of private data in the name of choice and control, they can be encouraged to do so unwisely. Users rarely stop and read warning messages, no matter how simple and clear and straightforward they are.
Not trusting user input is about security. Verify that everything is something which they /should/ be able to do. Think say for a game's bank account. They should never be able to directly change that value without valid transactions to and from.
If they can lose money by writing a check to null that is bad. If they can /gain/ money by writing a negative check to null that is /really/ bad. Also really bad is being allowed to just take money from banks of others without a validated transaction.
Configurability lets them do anything that is in their rights. If they can access their bank balance and set it as part of their signature that is allowed along with say setting their crosshair to display the target HP which they already know. If they can create a custom texture of their bank balance repeated ad nauseam that is fine. So long as they are capable of that stuff already.