I always thought about this too. I guess you can ask yourself then what you would rather do. I know its not the same, but people here make their own design and can be free with their skill set and then they sell their creation. As opposed to a client's wishes you are free to do what you like and what you want. Digit is right though, you are getting paid for what they want not you.

About adding it to your portfolio; that's totally up to your standards. If you are not proud of it then leave it out. After all a portfolio is your individual form of advertisement. If you want people to see you are a good designer, then show them that.