Well, its easier to upgrade from a Canon DSLR to Nikon DSLR in the future with 3rd party Nikon lens mount, unless 3rd party Nikon mount has a constant problem with Canon DSLR else i doubt why it shldn't be use.
Selling a lens to upgrade to another lens mount can oso mean losing about 20-30% $$, while using a 3rd party Nikon lens mount on Canon DSLR would also means i could use it on a Nikon DSLR without having to sell it away and losing a part of the investment ;p