Is frequently toggling airplane mode on/off harmful to iPad hardware?
Technically the answer is yes. However, the "damage" is insignificant. It is like asking if I use my iPhone to make phone calls will it wear out faster? And the answer is yes. But it is far more likely that your device will become obsolete long before it be worn out from constant "Airplane" mode toggling.

I would recommend putting your device in "Airplane" as much as possible to get maximum battery life from your device. Keep in mind you can have your device in "Airplane" mode and still be connected to WiFi, so be sure to turn off WiFi when not needed as this will also increase your battery life.