As described, running your iPad from its Power Adapter will do no harm - as the iPad will optimally charge the iPad battery. Running your iPad in this manner will do no harm, but instead will likely extend the serviceable life of the iPad battery by reducing the number of charge/discharge cycles over a period of time.
All batteries have a finite lifespan - principally determined by charge cycles. iPad batteries are rated to to retain 80% of their original capacity after 1000 full charge cycles - a full charge cycle being any combination of charges that total 100%. For example, charging your battery from 70 to 90% of its total capacity is a 20% charge; doing this five times equates to 100%, or one full cycle. Likewise, 60% + 30% + 10% = 100%
As you can see, running your iPad from its Power Adapter potentially reduces the number of charge cycles - and will therefore extend the usable life of your iPad.