MLB is trying to put out the best product they can for the consumer.
In the American League pitchers concentrate on pitching, the DH concentrates on hitting. The fans want to see the best pitching and best hitting for their money.
In the National League a guy that can hit well has to be put somewhere on the field to be eligible to hit. The fans don't want to see a guy in Left field that they know will make an error every time a ball is hit to them. They don't want to see a pitcher that can't hit come to the plate.
National League fans want a player that can hit/pitch or hit/field.
Both are good points. Interleague play allows both scenarios to be witnessed. It's OK to like AL or NL rules and complain about the opposite.
An old guy that can still hit can be put to use in the AL. Edgar Martinez is my point of reference for this.