I never had the ability to bill by the day, but if I head back to contracting will try it. By the time I learned about it, I didn't have new clients coming in the door to experiment with. And existing clients and I were okay with billing by the hour.

I think that it is unabashedly a pro, because whenever you are adding value to the business and can defend it in an invoice, you're able to get value for it. Unlike when you are a salaried employee and the extra hours don't get you anything material (maybe noticed and provided with a raise. maybe).