Usually, the only reason I take my laptop any more is if I want to offload photos from my camera faster than I could using my phone. Otherwise, my phone and iPad are enough.
Yes, the app stinks. You can access the Disney website using your phone, and go through the website that way instead of the app. You aren't strictly restricted to using the app with your phone. That way you can access the app, or the Disney website through your phone's browser. It will work the same as if you were using the computer (load the full version of the site, of course). Not as easy as using the laptop if you have a small phone, but then you aren't carrying the laptop, and can still make changes on the fly in the parks.