http://wiki.theory.org/BitTorrentSpecification#Integers http://bittorrent.org/beps/bep_0003.html tl:dr: THE BUG IS NOT IN UTORRENT, IT'S IN THE TRACKER SOFTWARE.
It is necessary because the BEncode specification says it's necessary. IT'S NOT UTORRENT'S BUG THAT THESE TRACKERS ARE IN VIOLATION OF THE SPECIFICATION FOR CHOKING ON SIGNED INTEGERS!
Why the hell should we fix a bug in THEIR code? Is it our fault their parsing code is in violation of BEncode specification? No. It's THEIR bug for not accepting signed integers even though the BEncode specification says they're valid.