There are limitations with both SQLLite and SQL Compact that may require something more capable like SQL Express. For instance, what if the app involves reporting and data analysis? Most tools support attaching to SQL Express, but not in-process databases. It seems that there are a lot of real-world business scenarios you're not thinking of.
If you're going for edge case business scenarios as your rationale as to why RT isn't a good choice, perhaps you should be considering whether Windows8-Pro is more appropriate for your business needs than Windows-RT.
Windows-RT is never going to be a good fit for where you're forced to use desktop applications, supporting services or plugins to existing applications and can't provide equivalents as a web-service.