Hmm, I jumped in here to say that sometimes the format actually need to be correct, and you have no choice but to validate and reject if wrong. But then I took a look at the app I am working on, which has, currently, lots of validations (and validation bugs. It's very alpha, yet), and I couldn't find a single field where a better form control (that naturally limits the user input) or a sanitation function couldn't do a better job. So, maybe you're right.
Any way, I got some food for thought.