Protecting some of the 2.5 is to insure you have enough bucks reaching the older age class. It does little good to implement QDM on a property and pass the 1.5 y.o.'s,.. then turn around and overharvest them at 2.5.
Example....
You have 640 acres you manage. The current buck age structure on the property is 5 yearlings, 4- 2.5, 2- 3.5 , 1- 4.5. You protect the 5 yearlings,.. the 3 bucks 3.5 plus are legal for harvest,.. but you need to protect at least 2 of those 2.5 to insure you have bucks reaching the 3.5 age class next year. Why not protect all 4 2.5 ?? You can,.. but keeping realistic goals and to keep X number hunters satisfied,. you must allow for some 2.5 y.o. harvest.