Is this a reason to avoid this lens? If so..why hasn't Canon fixed this?
Neuro explained it rather well. And I understand wholehearted the concern anyone would have about this lens. It seems there are always chatter about focus shift issues with this specific Canon lens. That said, I just want to say that I have no problem whatsoever with this lens. I use it with my Canon 5D Mark II and focus has never been an issue. Never. One can argued that I am either naive, don't know any better, or too dumb to know what focus shift is. Or one can simply say that I am one of the lucky few that some how got a hold of a perfect specimen. Whatever the reasons are, I can say that I am one happy 50L user...and you will find many of them around.
If you're truly interested in the 50L, my best suggestion for you is to simply purchase it from B&H as they have a great return policy. If you find problems, then simply return it. Spending any more time researching isn't gonna make it easier on your decision. If you truly want to find fault with a lens, you will.