Browsing another user's shared files would be nice, but how would that help me find that particular user ('s files) the next day, unless he had a static IP?

Would sending the information "myaddress.dyndns.org" along with my IP and agent version (i.e. Xolox, Limewire...) really use that many network resources ?

The dynamic DNS service itself would not be hosted by Xolox. Xolox would only use the information (user myaddress.dyndns.org has file "y").