This seems like a HOT issue for most Tableau users who receive files (incremental or full extracts) on a regular basis ex. from Surveys, Marketing, Operational etc. and want to perform analytics on the entire file. I am aware of few options out there (UNION using multiple sheets in an XLS, merge files outside Tableau manually, etc.), but sometimes that is not a possible option due to limitations.
Most of the times, files come in same formats and having the ability to UNION these files to get a unified data set, would allow users to leverage Tableau for analytics from start to finish.
Even if the files have different structure, having a UNION option solves the issue too.