The problem is that the type it infers is not necessarily the type you want to be exposed on that export. And then it's quite possible to have a bug in implementation of the function that results in a type that's outright wrong.

Types are a subset of contracts. Contracts are best explicit at the API boundary - which is to say, on exported functions and members of exported classes.

Agreed—there are times you definitely need to specify the type. I have them every week, but still find it the exception rather than the rule, and easier to adjust for those, than adjust for the more common case.