It's probably the simplest way of measuring just a guys power. It's not an aggregate measure of production in any real sense, so don't think of it from a value standpoint. It's most useful as a skill estimate.
Games are won on run differential -- scoring more than your opponent. Runs are runs, scored or prevented they all count the same. Worry about scoring more and allowing fewer, not which positions contribute to which side of the equation or how "consistent" you are at your current level of performance.