This is the 3rd time I've posted this answer in this thread.
ALL 3 core sets share some universal cards (i.e. Hold Objective 1-5 cards, Annihilation, Conquest, etc Objective Cards as well as Confusion, Great Strength, Great Fortitude, etc power cards.
Since these cards appear in all sets, they WILL NOT cycle out and can be used with any warband.
With those and the faction cards, you can make a legal deck for any faction you want for the life of the game without EVER buying another pack.
Many people hope/think that GW will re-release packs for old factions that include updated abilities and a full 12 card faction objective deck (they originally only have 9 faction objectives) to match those in Beastgrave (all warbands going forward will have full faction playable power and objective decks).