There will always some people that will get to the end without getting their CFI. But just having the cert can open up a lot of doors, and it may be the best way for you to hit a milestone like 500 hours.
As far as not being good at teaching, I would wager to say that most CFI's don't start out as 'good teachers'. But they will do alright if they have a passion for it and truly care about their students. Then, the more they teach the better they get. Now I feel really bad for the students I had back when I was <100 hour CFI, because I realize that I'm so much better now than I was then.
Then there are some people that really aren't cut out to be a CFI. IMO, from CFI's that I met that seem to fall into that category, a lot of them never wanted to CFI and weren't really willing to try to make the best of it, they just ended up being bitter that it was a step they needed to take. That is going to be one of the sad consequences of the ATP-rule, a lot more of those types of people will be stuck instructing and screwing over students.