I think it could be useful. I wouldn't be worried about devs closing issues prematurely - there is nothing to gain unless Bangle app devs start monetising the apps somehow.
I think the question about how they'll be counted is sensible though. Issues in the BangleApps repo could be tagged. As @andrewg_oz says it is easier to keep track of bugs on a separate issue tracker per app, but not all apps have separate repositories (not sure if it is even the majority, and I understand why).
I'd personally love to see more screenshots in the app loader, more detailed descriptions, and more apps with an upload to emulator button (not sure what decides that currently, and I probably won't care as much when I can access the real thing).