If I do this, I'll only have to submit the iOS version of the app and not have to dork with two app reviews. What do you think? What am I missing?
The idea is that the developer is expected to provide a better user experience if you use Mac Catalyst and put in the time and effort to make your app feel like a "Mac App" (by that I mean an app that behaves as close to an AppKit app as possible, though Mac Catalyst creates some arbitrary barriers that does not always make this an easy task).
If you let your iPad app run on Mac unmodified it's definitely easier for you. It all depends whether or not you think your app is "good enough" for Mac as an unmodified iPad app. It won't run on Intel Macs though.