Well, look at Kar Vastor. Untrained regular schmuck, ends up as a super-dangerous badass. No one but a Jedi could have taken him down. That's why you need Jedi. There have to be trained Force users with discipline and self-control. Sure, you run the risk of trained Force users without discipline, but you can defeat them and the untrained Force users without discipline. It's like the difference between having a trained military and relying on an armed populace to keep order -- sure, a military could lead a coup, but it's a necessary institution, and you just have to minimize the chances of something going wrong.