Fortran intent(inout) versus omitting intent

Good practice dictates that subroutine arguments in Fortran should each have a specified intent (i.e. intent(in), intent(out) or intent(inout) as described this question):

subroutine bar (a, b)
    real, intent(in) :: a
    real, intent(inout) :: b
    b = b + a
    ...

However, not specifying an intent is valid Fortran:

subroutine bar (a, b)
    real, intent(in) :: a
    real :: b
    b = b + a
    ...

Are there any real differences beyond compile time checking for an argument specified as intent(inout) and an argument without a specified intent? Is there anything I should worry about if I'm retrofitting intents to older, intent free, code?