Well, in a general stand-point of view, this all comes down to what suits your needs.
XDCC Example:
I met this college student on IRC that couldn't download through BT because of application restrictions! (happens a lot .... damn colleges)
Bittorrent Example:
For some people IRC is too much of a hassle to learn and use or some people don't have the nerve to wait in line for their download.
Summary:
The more I think about this, BT and XDCC sorta cancels out xD
This is simply a INTERPRETATION! I'm not rooting for either software
