I guess it might make sense if you're on the move with both the iPad and the MacBook. The monitors aren't portable. But at home or at the office I agree that a monitor makes a lot more sense!
Actually, you can get a really nice portable monitor for travel use... and they are not very expensive.
Here's an example: https://www.amazon.com/gp/product/B013XFJKGI/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1
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