Here is some food for thought, as I also reflect often on the value of screen protectors. With the exception of working in sandy, dirt environments, then the value of them can be questioned.
My flow chart of analysis for scratch protection
1. Never scratched your plastic film screen protector? Then you definately
don't need a SP. These films have a hardness of 3, the iPhone glass is 9. If you're not scratching the film, you will never scratch the glass.
My wife falls into this one. She has a bumper and plastic film. For a year, there have been no scratches on the film.
2. Never scratched your glass protector (any type of glass), then you probably don't need a SP either. The Zagg is the same hardness as the iPhone screen.
Shatter protection: In general SP vendors downplay the benefit of SP for shatter protection. With good reasons, I believe. They don't add much value.
1.) if the SP, like the thick glass ones, reduce the case's front lip protection, you have reduced the protection of the SP too. Both scratching and shattering. I've seen thick SP on cases, where there is no lip left at all! Stupid!
I've read some people claim that the glass SP protector broke, protecting the iPhone glass. Possible, true. Most likely the thiness and poor fitting of the SP caused it to break sooner than the main iPhone glass. Give any wiggle room, or play between the iPhone glass and the SP, and it will break there first...especially true of the poor fitting edge to edge protectors. Or poor alignment of the SP that just cover the flat iphone screen.
Think about it this way...place you finger under a SP, hit it will a hammer...still hurts, huh? The energy is still transmitted to your iPhone screen...the SP is too thin to aid in shatter protection.
Each persons usage is different, but this is what I think about. I mostly have not used a SP since the 3G and no scratches or shattering. I use cases like Apples or Tech 21, or Spigen bumpers. For really bad envirnoments I have had LifeProof cases.
IMHO, I rather add my iPHone to my house insurance, or get AppleCare+, then pay the money ($20-$80) to replace a SP. Yes, accidents do happen. But the look of SP is not great. I rather go with insurance, given my life style.
Screen protectors like glass ones may add protection if you are scratching the SP, and it is not due to reducing the case's protective lip. I doubt they add much for shatter protection. A good lip is a better choice.