For non-paying users, MF's file size limit is 200 MB, whereas MU's was 1 GB. If you knew how to concatenate files, this wasn't an issue, but the vast majority of people would have difficulty even grasping the concept.

RAR (both with the official client and 7z) makes concatenating effortless; you just dump all the files in a directory and extract the main one.

I wouldn't say effortless. If you're sharing with fellow tech-savvy people, it's fine. But otherwise, the 45 second wait for MU is much less than how long it would take to explain the concept of split RAR's/7z's if the person wasn't familiar with them.