Do rear brakes serve any purpose?

No, it wouldn't be safe. My car is equipped with a dual-brake system--in other words, the braking system on the left wheel is connected with the rear right wheel, and vice versa. That way if one system fails, I will still have braking on two wheels. So to answer your question, yes it would be dangerous if the rear wheels didn't have brakes.

Buy a pedal cycle. Get up to full speed. Put the front brake only on and see how far it is b4 u stop.
Repeat and use both front and rear brake. Any difference?

yes.. they do about 25% of the work
and you should have brakes on all 4 tires
you'll see an improvement!