Why would they be a disadvantage? For every "easy" winner in the 1st half like KC/Chicago, you've got lots of upsets such as Indy/Houston, Zona/Minny, Cleveland/Cincy.
Some favorites come out blazing in the 1st half. But some favorites sleep-walk until the 2nd half.
As long as the books get equal action, they'll offer bets on ANYTHING.