Unless you're able to have an interrupted block of time for each project -- a worthwhile goal, but in our multitasking world, a tough one to achieve -- billing by the hour runs the risk of being an unethical business practice.
It might seem churlish to single out Barclays for this criticism, especially since I know that virtually all the other major banking institutions involved in money-laundering, LIBOR manipulation and all the other unethical and illegal activity over the past five years, have similar codes or principles of best practice in place and claim to link them to remuneration.