Final arguments in interface methods - what's the point?

It doesn't seem like there's any point to it. According to the Java Language Specification 4.12.4:

Declaring a variable final can serve as useful documentation that its value will not change and can help avoid programming errors.

However, a final modifier on a method parameter is not mentioned in the rules for matching signatures of overridden methods, and it has no effect on the caller, only within the body of an implementation. Also, as noted by Robin in a comment, the final modifier on a method parameter has no effect on the generated byte code. (This is not true for other uses of final.)

Some IDEs will copy the signature of the abstract/interface method when inserting an implementing method in a sub class.

I don't believe it makes any difference to the compiler.

EDIT: While I believe this was true in the past, I don't think current IDEs do this any more.

Final annotations of method parameters are always only relevant to the method implementation never to the caller. Therefore, there is no real reason to use them in interface method signatures. Unless you want to follow the same consistent coding standard, which requires final method parameters, in all method signatures. Then it is nice to be able to do so.