Postel's law doesn't mean "accept everything", but that you should accept de-facto rules people have created. If everyone says, "this is how we do it", you should ignore the RFC and just copy what others do.
One, if everyone is doing something different from the spec it is hard to figure out what they are really doing and what they mean. Long term you have confidence things will continue to work even when someone else writes their own version which otherwise might also deviate from the spec.
Two, it is easier to modify the spec as more features are dreamed up if you have confidence that the spec is boss meaning someone else didn't already use that field for something different (which you may not have heard about yet).
Three, if you agree to a spec you can audit it (think security), if nobody even knows what the spec is that is much harder.
Following the spec is harder in the early days. You have to put more effort into the spec because you can't discover a problem and just patch it in code. However the internet is far past those days. We need a spec that is the rule that everyone follows exactly.
