I agree this is a feature that would be great to have.

We have created some custom transaction reports where we added the client name. We have one of these for assessments that we often bill the client for, and another for our service travel category that we normally bill for. This at least lets us know if we allocated it to a client, and if not, we most likely need to look at it to allocate and mark it as billable. It would be great if the billable flag was available to show in that report so I didn't have to click open each one to verify we billed for all of them.