Like what many had mentioned about the quality, resale etc...going back to your question, from the opinions of a decade old 35mm photog, have you asked yourself why did you choose Canon in the first place?
Most 35mm users, be it users of manual film or auto digital, stick to the 3 major brands namely Nikon, Canon and Minolta. While we see that Minolta had went through 2 takeovers/mergers, I'm expecting the Sony Konica Minolta team to come out with a very competitive system to battle against N & C.
Going back to why choose Canon over the rest, I dun suppose Canon system is cheaper than Nikon or Minolta right? From what I know of old manual film users, photogs stick to Nikon because they have a better range of accessories under the 35mm system. People who choose Minolta wanted a better flash system but having limited accessories makes their users limited.
In the late 90s to early 2000, Canon caught up with more lenses and better technology that works - targeting on the increasing market for photo-journalism, sports, nature and aerial photography. New photogs at that time started taking a look at Canon lenses instead of following their elder brothers, uncles or dads who are old Nikon advocates.
Most 35mm users choose Canon during the time when the L lenses flourishes and peaked when EF and IS comes in. Later when digital comes into the picture, it make sense these to migrate into this new platform because of the system they have painstakingly build over the year. To illustrate this point, most renown photographer like John Shaw, Arthur Morris, Moose Peterson etc are still advocates of the brand of their film system.
I dunno about you, but my decision to go Canon is basically because of their lenses arsenal. If I would want to opt for 3rd party lenses, I would be more happy to go for Nikon and Minolta bodies because they are much more interesting than Canon's. Film users would have remember Nikon N90, F4, F100, F5 bodies, and the undeniable Minolta Dynax 7 & 9. Canon bodies comes into the limelight with the EOS 3, 1v and the much later 30. With Nikon's VR, ED technology comes years later after Canon launched their EF, IS and L technology, Canon is undeniably a hot favourite.
Coming to the digital platform, my friends, myself and many film-turn-digital photogs, who have been decades old advocates using only Canon lenses and spending a fortune over them - also tempted on whether should we switch to other brands when digital evolves. Nikon wins back many of the young digital shooters because of their constant commitment on camera bodies that had make it more worth while Canon continues to fancy us with their lenses and upgrades them constantly. Just imagine in 12 years, some lenses such as the 20-35 f2.8L, 80-200 f2.8L had been upgraded 3 times and the 4th one coming near! None of the other brand lenses get an upgraded model every 4 years and its also because of this consistent effort, Canon lenses have been able to impress users to stick religiously to them. (Ironically, the word Canon is actually a derivation of the name of the goddess of mercy)
So at the end of the day, unless you find a very good offer to start with or have other personal reasons such as sharing lenses with your friends etc, to get a Canon system, IMHO, is only worthwhile because of their lenses and their system. This is especially true for films because film users focus almost 70% on lenses for their works while their film bodies are not as feature packed as a digital model.
So whether to justify getting Canon only lenses or not, depends a lot on how much you value their lenses and your understanding of the Canon system. Relying and debating on 3rd party techincal tests to match Canon's won't go far as we all know that if $$$ is not the concern, no one would choose a 3rd party over a canon.
