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.
Apple made big improvements with the latest iPhone screens. I've had the iPhone 4s since for a while now (since it came out) and I'm notoriously hard on my phones, but the screen still looks as good as new. Personally I find the plastic screen covers just annoying, they always come off and they're a pain to apply. Hope that helps.
I've been using a transparent protector since I had my first iPhone (4 years ago) and I can only recommend it. If you buy a decent one, you won't even notice it's there.
I agree Apple has made a lot of improvements on the durability of the iPhones, but the protectors has always kept my iPhones next to brand new (which has come handy when I sold my iPhone).
And these days, you can buy 10 protectors for less then $10, so in my opinion, this is a very small amount to pay to keep my iPhone scratch-less.
I agree that screen protectors are quite necessary, but not only for just resisting scratches. The ones I have used for years (ScreenGuardz) have an anti-glare feature which is nice to have in bright situations and they are less prone to collecting fingerprints all over the screen.
The decent ones cost a bit more (still inexpensive) but are still worth it. Here's the ScreenGuardz link...
Check for them on eBay though, they might have new ones for cheaper. ;)
But, if you have already picked up scratches on your touch screen panel, then make sure to use the right buffing agent for your specific screen to work out the scratches. Touch screen panels vary in their composition (Glass, Acrylic, Gorilla Glass, etc.) so be sure you use the right one. These guys have a line of buffing compounds specifically for mobile device touch panels so you might look them up...