Prior to the BCS it was extremely rare to teams outside of the power conferences and Notre Dame to get access to any major bowl. There were no guarantees. When the BCS was created a major bowl would HAVE to take a non power team if they finished in the top 5. Then it was lowered to top 6. Then it was lowered to top 12. Now the highest ranked G5 champion is guaranteed a spot in a major bowl regardless of ranking.
If they expanded the playoff to 8, expect the 6 highest ranked conference champions and 2 at large.