Can the 5D be used for sports photography? Sure. Heck, if you are really good, you can get great shots with MF, using the right techniques. The question is, what is your keeper rate, and can you (not some unnamed maestro or surgeon with butcher knife) capture the precise moments you want?
Good skills are important, and will allow you to optimize the technical capabilities of your equipment, and achieve results that one with lesser skills may not be able to.
But I think picking the 'right' camera is sensible. Even the best photographers would pick the most optimal gear for the job at hand to get the best results. If you only have one camera, then the question is moot n u just make do with what you have. For me, my iPhone is my best camera sometimes.