If you don't fly much it might be better to pay the $50 guest fee to use it on an a la carte basis. If you join and are plat. the renewal is only $300 per year.
My wife would never let me give up my membership even if she used it once per year. She likes the bathrooms!!
40k miles are too valuable to me to sell for .01 per mile. I opt for shelling out the cash.