I ran into a very good reason to backup the original MXF files. I shot an entire day of b-roll, and was logging and importing the shots one by one in FCP (import P2.) After I did this, I realized that some of the files I had renamed, but never clicked on the ADD TO QUE button. So, those files did not get imported/translated. I was lucky and noticed this before I had a chance to delete anything. I easily could’ve assumed I had all my shots and deleted the originals to save space.
Also, I had created quite a few long named (too long) for these logged clips… and they did not translate properly. They came across as 6k files.
IMO – hard drive space is really cheap… so I will play it safe and always save the original files.