I've been using Duet app intensively lately. The app lets me to use my old iPad 4 as the external monitor for daily use. Is it ok to keep the iPad plugged into the MacBook Pro all the time?. Will it add any new kind of stress to MacBook Pro?
every few weeks (3 or 4), disconnect the iPad and use it until you have about 50-75% battery left, then charge/connect it again and your battery will not suffer at all.
Actually, frequent charging habit is good for the Lithium-ion battery use device.
You could refer this Apple official page. http://www.apple.com/batteries/why-lithium-ion/
You shouldn't be worried about your Macbook Pro, more the iPad. Having the display on all the time and not using the battery might eventually 'damage' the iPad.