This is why I think that's wrong: if you allow people to be sloppy with how they do things, they'll do it, and then make it part of their workflow, product, religion or whatever, and now everyone is stuck with it.

Be absolutely explicit with what you accept and refuse to deal with crap. Then you will only ever have to maintain a simple validator and the code that deals with good data, rather than having to have an incredibly hairly validator that leaks into your logic at every level, followed by cementing your bugs into everyone's implementations.