My opinion....
Obviously they are going to stop the case being scratched
Yeah, that's probably true. But what about the glass side ?
but if the damn thing falls to the ground will they stop the smartphone case cracking ?
Probably not.
To be honest, any decent insurance policy (not the ones the carrier's try to flog you, but your home one) should cover loss and damage anyway ! The insurers might prefer to get it fixed rather than replaced.
My personal attitude with electronics is that given the rate of R&D industry, and the volumes in which the kit is sold, the depreciation in value is extremely quick, and within a 3 year cycle you are looking at an obsolete piece of technology. Therefore I say use it, abuse it..... and by the time you properly break it, it's probably already time to compare the cost of fixing it to getting the latest model.
Most kit these days is surprisingly robust anyway, I've dropped my MacBook Pro from desk height onto a concrete floor (after a quick airborne flight) and it's still operating perfectly apart from one or two cosmetic dents....