Is frequently toggling airplane mode on/off harmful to iPad hardware? I want to try to save my iPad batteries, so I turn on the airplane mode when I don't use the iPad during the day and at night. I could toggle airplane mode on/off for more than 12 times in a day.

Do you think it's worth it to put iPad into airplane mode when I don't use it during the day? and is frequently toggling ipad airplane mode on/off harmful to iPad hardware (shorten hardware life)?
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.