The big advantage is to Airbus. Airlines use the choice of engines to make GE and IAE compete for the business, and it makes the Airbus more competive vs. the 737, where you have no choice. The Airbus competition gets to the point of giving the engines away.
Boeing are not thinking of competitive advantage, when they offer a choice of engines only on the 757 and widebodies. They would sell more 737 if they offered the IAE as well.
GB