I do it because I'm not concerned about the value. I don't collect because of the value. I like to store my cards in plastic pages - as God intended
. Because I'm a type collector and not terribly interested in having the most expensive cards in the highest grades, I can justify it easily to myself. If I had a 52 Topps Mickey Mantle PSA 8 - sure, I'd leave it in the plastic. But, I'll never own such a card because I have no desire to have investment grade cards.