In order for the measuring tool to be truly useful, I feel there are two critical changes that need to be made to it:
1. The unit of measurement needs to be adjustable in some way. Most useful would be a simple counting of the number of hexes/squares. While it would be nice to be able to set a specific distance per square/hex, many systems count the squares themselves (and it is easy to multiply a number of grid spaces by a set distance).
2. The measuring tool needs to have an option to be persistent and visible to all parties. If a measurement can be made, seen by all players and the GM, and the measurement stays visible until the acting player finishes their move, it would go a long way to improving the usefulness of Roll20. It would also open the door for a number of tabletop strategy games.