Of course, it's added value. If you sell a design as just the psd then it's only worth the price of the psd. The client buying it would have to either spend their time coding it or hire someone else and that costs money.

However, with that being said coding up a design and integrating it into a script will also cost you time and time is money! I agree though, you should always sell a design as a finished product (much more chance of selling than just a psd)