29" wheels on MTB's are actually used because they 'span over' dips and bumps that would possibly swallow a 26" wheel. There is little if any centrifugal advantage to using a slightly bigger diameter at the rotation speeds turned on these bikes. It's more about increasing your contact patch.
There is a downside to making spoked wheels bigger, they become weaker and are less able to hold their true when subjected to abuse.
We're here because of a love most exestential. (toothy)