Consider:

module Main() where

import System

data Foo = Bar | Baz
           deriving (Show,Read)

main = do
    [x] <- getArgs
    print (read x :: Foo)

When we compile:

$ ghc -c Test.hs -fwarn-unused-binds
Test.hs:6:11: Warning: Defined but not used: data constructor `Bar'
Test.hs:6:17: Warning: Defined but not used: data constructor `Baz'

This is incorrect. If a data type derives Read, Enum or Bounded (and for Bounded, is either the first or last element), then the values are used - even if not by name.