Well, there are a few reasons one might temporarily wish to use ports below 1024.
The most obvious is for a sender to circomvent a receiver's firewall restrictions... ie, office computers that can only access port 80 of any internet host. Temporarily setting your DCC Send port to 80 would allow this user to "get" from you. (IRCing from work can be difficult at times
There may also be other, more legitimate reasons to doing this.