I always recommend that people use the "Working On" feature, usually in conjunction with the AutoAssign plugin, rather than manually enter time. You can even find a System Tray application to change your "Working On" here. The benefits come down to efficiency and accuracy; you don't spend time editing your timesheet and you don't risk messing up your EBS through typos.
To answer your question, though, I think this fits better as a plugin rather than a native feature. We have to carefully balance the complexity of FogBugz, both in code and in user experience, with the value that each feature provides. This is twice as important when new features introduce new settings and new knobs to twist. While this feature adds value, I'm not sure it adds enough to counter-act the complexity it would generate. As a plugin, however, it can selectively add value to only the people who want to see it.
But maybe I'm wrong! We're always willing to take a second look at a feature that gets a lot of up-votes or that is frequently requested (make sure you up-vote the question and not the answer). Many of the features we implement start as suggestions posted here.