CRM may just be "common sense" (as IO540 claims) but the trouble with "common sense" is that it's not very common (as IO540 proves regularly).
One objective of CRM is to fast-track common sense into inexperienced pilots and then, as Ghengis correctly points out, refresher training proves to you that you never stop learning and reinforces the lessons learned previously.
Sadly IO540 would appear to be just the sort of pilot who would benefit from CRM training - because he thinks he knows it all. Equally sadly, these know-it-all types are usually the ones who are not prepared to embrace CRM because they think that they've learned all that there is to learn! It is a fact of life that experienced does not always equate to competent and in my previous position at an FTO based on an airfield used by lots of owner-pilots that fact was reaffirmed almost daily.