It costs like $30-50 bucks to replace the back glass. $100-150 to replace the front glass. (from local stores here at least)
According to a sampling of 20000 iphone4's and iphone3gs' (20k each model), 771 iPhone 4's had broken/cracked glass and 427 iPhone 3GS' had broken/cracked glass. The glass isn't weaker, it's just more glass to crack.
There is a lot of myths and inflated anxiety out there as case companies are trying to make money by instilling fear into you. Those prices and odds are better than having to have your phone look stupid. (you are essentially wearing the fanny packs of the iPhone community

)
EDIT - My issue isn't with the skin but the horrible case bulging on the sides.