Most likely the Brake Light bar has all the LEDs wired together, for all On or all Off operation. Displaying a bar graph would require individual control of the LEDs, so an expensive retrofit of every Tesla. And, when looking for a spot, where most cars back in, the brake light bar would not be visible. Oops, now I see a new post that came in while I was typing. But I agree.
Being able to query the state of each supercharger location is already available by tapping the location's icon on the Nav map on the touchscreen. The only problem is ... the minimal status given on the popup is rarely of any use ... that the location is off-line, or not.
Given access to Tesla's supercharger database, it would seem that very useful detailed status could be presented about every stall ... charging, time to done, done charging, or apparently empty. Users of the system could even indicate when a stall is blocked, ICEd, broken, etc. by clicking a checkbox ... crowd-sourced additional data that Tesla cannot detect about the stalls. Also, add some checkboxes (or a number) indicating the number of vehicles waiting in line, ... that would also be very helpful, I think.
Then, if your travels give you any flexibility with when and where to supercharge, one might be able to check the current status of the superchargers before committing to get off the freeway.
Tesla would also get some real time data that it does not have now, especially about blocked stalls. In some busy locations, around half the stalls are marked as shared parking, although usually with a limited time constraint ... like in San Diego.