It would make the game more fun if they actually had to look over their shoulder and make a decision. Over-coaching is boring and takes away from sports.

your back is to the action so they are your eyes and then they know what you should do slide,stop,or run home.

Because they cant run at full speed and watch whats going on behind them at the same time.

If the ball is in right field and they are rounding 3rd, it helps to have someone watching what's going on. I don't anyone that has eyes in the back of their head.

their coach tells them whats going to happen when they cant see and are running.

but if they have to look over their shoulder, it would make the runner slower, which results in less runs

so they don't need to turn around and look at where the ball is! If they round third and the ball is coming to the infield, you can be picked off at third. Why not have an extra set of eyes.
If your talking about changing the rules though, that sounds interesting. Would definatly put a twist on the game!

they also help the batter on what to do

it makes sense to have coaches, as it is often hard to judge a ball hit into the outfield by "looking over your shoulder", also, how often do players make stupid running mistakes WITH coaches, imagine those runners with no coaches at all

I have a question for you.....Have you ever in your life played baseball with and without one. You need someone to tell you when to stay up, get down, or go home. If you looked over your shoulder two things can happen. You will slow down and you may hurt yourself by tripping over your on two feet. The third base coach is also responsible for sending in all of the plays.