This feature would be fantastic. I'd love to be able to generate a billing report for my clients based on time worked in the system. I can do this now but have to massage the data in Excel after downloading it to add the hourly rates. If the hourly rates could be built in this process would be a lot easier.
Of course, in an ideal world, I'd be able to set an hourly rate for each user in the system, but then override that rate for individual customers, workspaces or tasks. But I know that would be pretty tough to make work!