Crop Rotation

I was talking with the neighbor this evening and he was tell me that he thinks we were nuts for not putting our hay field back into alfalfa this spring.  We were told by the local CO-OP to rotate to something else, so we planted oats this spring, and plan on planting RR alfalfa next spring.  One thing about this neighbor, he is a know it all, and he has never planted anything but alfalfa for the last 10 15 years.  What are the advantages to rotating to something else for a year?   

Crop Rotation