I definitely think it could be a good idea to have both options, mirroring and second display, just because I know people have different preferences. Personally, I'm always going into preferences trying to find if there's an option to change it to a dual display because I just need more screen space!
I purchased this app, an ipad, and the adonit jot 4 instead of buying a cintiq but the main things I feel like I'm missing are:
1. The ability to have two monitors, at school I am always pulling up pose references and inspiration on my computer while drawing on the cintiq, for the most part all I want to be able to see on my iPad is my drawing program, besides the occasional time when I bring my reference onto the screen so I don't have to keep glancing over.
2. Palm rejection (My lines often get cut short because my hand bumps the screen, with long sleeves being my only savior. I know it has some palm rejection but I do feel it could be improved)
3. A little more line quality and more sensitivity to small detail work. I've found that my line quality in apps like procreate are stronger with my pressure-sensitive pen than the astropad app.
I love the app so far but I am very excited to see what a new update will bring! I'm hoping it's coming soon! And if this app keeps improving, who knows maybe I won't ever get a cintiq and I'll just upgrade to a bigger and better ipad with this app for my in-home setup!