Picture this scenario:
A file limit is added. Now Joe Warezdaddy splits the big file into RAR volumes, each the maximum allowable size. Now not only does that not slow down the use of large file trading (and remember that large does not necessarily mean illegal) it does in fact mean that when DCC resumes fail (which is quite often in my experience) these people sharing the large files now only have to download that particular volume again. The sharing of large files has now been inadvertantly forced into being more reliable. Whoops.