Heat and wasted energy. The ignition only requires 4.8v. Anything beyond that is shed in heat, which would not be present at the lower voltage.

As for battery duration, some of us have been doing gas for a great many years and learned by doing, not by reading what someone else said to someone, who told it to someone else. Jody is right on about how long an ignition can run on "X" size battery. Most of the newbies have about 4 times more battery, and weight, and cost, than they need to run the ignition. An 800 mAh battery can usually get in a full days flying on a single for most people.