Battery life, I understand, is dictated by charging cycles. Batteries are limited in the number of times they can take a charge and discharge.
Using an battery-powered electronic device such as an iPad while it is being charged simply consumes energy that would otherwise be available to charging the battery. As such it is reducing the number of charge/discharge cycles. If the charger can provide enough power to meet the needs of use and have extra capacity, the battery will continue to charge, albeit at a lower rate than if the device were not being used. Given this, I would argue that charging an iPad while in use actually increases battery useful life because it is reducing the number of charge and discharge cycles than would be needed if one always operated on battery power alone. Regardless, the benefits, if any, are probably minimal.
Like the others, I say go forth and use while charging without worries. Just enjoy it!