For one there's no way to add a curried parameter without doThing4-style naming and lack of named arguments implies you can't have a default value for the new parameter.
Another one is if you want to add a curried parameter to the end of the parameter list, and you have code like
|> myFunc a b
|> ...
|> myFunc a b z=10
|> ...
OK, I think I see what you mean. I certainly agree that named arguments with default values can be useful, but are not supported by curried functions.