ESVs are usually plugged into the charging port on the switch harness. This port is cut off when the switch is turned on so there is no reading of the actual system. If the system is accessed by another method then yes the ESV will show the buss load on the system at that point in time.

The more inexpensive LED indicators only give gross indications but they also act as a secondary reminder that the switch is on. Sometimes this is worth more than the actual charge state.