Squaretrade seems worth it if you think you might break (but not lose) your phone more than once. If you break it once, Squaretrade is slightly cheaper than Apple's out-of-warranty service. If you never break it (the outcome for most users), you obviously come out ahead relying on Apple's OOW service option and forgetting about Squaretrade. All in all, Apple seems to be the best cost-risk balance.
The total cost-risk using Apple's OOW service becomes even lower if you couple it with free 90-day accidental damage/theft coverage by purchasing the phone using an Amex card or equivalent. I'd argue that handling an iPhone is an acquired skill and you're most likely to drop it in the first few days/weeks of ownership. I dropped my iPhone (on a tile floor) only once in the three years I've had them and it was the day after I got the original iPhone (the glass didn't shatter). Also, the first three months of ownership are also the time when people/family members are most likely requesting to see and play with your new iPhone. After that, it ceases to be a chick/dude magnet.