1. I have not heard such assertions and am interested in hearing where that claim originated and the research/facts supporting it.
2. Uninstallable system apps is nothing new in software. iOS doesn't let you uninstall Game Center (I think they did end up removing the feature or something), Mac OS X won't let you uninstall Automator, Palm OS won't let you easily remove Contacts, many Android phones come with uninstallable apps baked in that aren't even system.
3. I would not believe that claim for two reasons.
- Why the Xbox app? It's already running on Windows that Microsoft also makes. Why not add whatever backdoor or spyware into the OS itself or as part of some component that's more critical than a feature that has to do with GAMES? Microsoft already has a system/systems for analytics and whatnot, so why not just use that? (and the Xbox app requires Windows 10 and won't run on 7 or 8.1.)
- Without any presented facts other than an alleged "lot of people" there isn't much to say.