Does using a transparent screen protector for the face and the back of the iPhone 4 have drawbacks? I saw multiple videos that the iPhone 4 screen can not be scratched by keys, coins and is even quite resistant to dropping. What are the drawbacks to adding this layer of protection (in general or perhaps for a specific film you have used) so I can weigh whether I wish to use one?
In the end, the worth of a case/screen protector is entirely up to you.

The iPhone 4/4S screen is incredibly durable, and without a protector, I've had mine since the release of the 4S and it's only got the smallest micro-buffs in the screen (you really have to be looking at them to see it).

I'd say the only real good reason for protection of any kind is to protect your device from drops. Being that they're a specially treated glass, if you drop it from a sizeable distance, they'll more than likely shatter.