1) No, they don't cover accidental damage without insurance
2) somewhat, may divert some of the blow to the plastic case, and MAY require a higher blow to crack the screen
3) Not really, just annoying, and lint gets caught underneath them
4) Won't prevent shatter, will prevent you from giving yourself a shave if you want to use the phone with a shattered screen, and prevents your fingers from being cut up. Phone may still be usable with just a shattered screen.
For me:
I have to wear a shirt/tie/slacks/belt to work everyday. I currently use a universal beltclip case and haven't had any problems with accidental slippage. I did crack the screen on my current hero, but that was trying to get it out of a homemade nav cradle, which has a kung-fu grip on the phone. Had I taken my time taking it out with 2 hands, no issues, but i was trying to be a hot-shot and do a 1-handed move to get it out and it slipped from my hands, hit the dash, then the seat, then did a nice gymnastics move onto the concrete right outside the car. When I had the pre, my favorite case was the seidio belt-clip with the spring-loaded release. It allowed a tight grip and easy 1-handed release.
For this phone, since i won't really be able to use it much during work, except for looking up drug interactions with my pharmacology software (no phone calls allowed, or texting), I won't be in a hurry to get it out of the case, so any tight grip will work for me.