Your core muscles are your big muscles: chest, legs, etc. They call these the core muscles because these are the ones that need to be developed no matter what else you do. Its kind of like the core classes in high school or college; these are the ones that you have to take. Many of the isolation exercises are just extras. Using that analogy, you can consider concentration curls as kind of an elective.
-------------- --JBennett "I've up-ed my intensity.... now up yours!" "Pain is only weakness leaving the body."