It depends on a lot of factors:
The Cost of the phone.
The cost of the insurance
The provisions of the insurance, will they replace the phone more than once during the contract period. Is the "if it gets wet" exclusion in the policy?
How careful are you with equipment? Do you misplace your phone frequently, drop it in the sink or go water skiing with it (know someone who did).
My phone is not insured.
My son's phone is insured.
The one difference about phone insurance from other insurance, and that may have changed with the new generations of smart phones, your information, contacts, notes, etc, are not insured or recoverable. I think in some plans you can pay for an automatic backup to a server. If you misplace or lose your phone a lot, a service like that would be better.