The best reason to continue working on the product as CEO is that you can make far better management decisions that way. Even if that 10-20% time you spend coding isn't as productive as doing sales or business development, it's essential to keep that one foot in product development so that you can prioritize correctly.
I'd be interested to hear of examples of successful companies (other than Posterous) where a founder is CEO and continues to develop. I would think that would be a very challenging balancing act.