My cousin's in a wheelchair now after wearing away all the cartilage from his knees through his running habit. He had virtually no warning of this until one day his legs just gave way half way through a 10k run. Nothing they can do for him.
It would be foolish to suggest this would happen to everyone who runs but is just an example of the potential risk.
Personally, I just don't see the benefits of exclusive running for the effort employed. It works a relatively low number of muscles, obviously mostly in the lower body but is extremely taxing on other body parts such as ligaments, tendons, bones etc. Much better, and far more fun to mix it up like I mentioned above. I've done gym training for many years and yes it becomes monotonous but have recently gotten into kettle bells (which is GREAT for the core and muscles you don't often use) and circuits (which is loads of fun and is easy to mix up the routines).