I use a similar approach. The insurance companies are in business to make money, so (on average) they don't sell policies that lose them money.
I buy insurance if either:
1. I believe I'm significantly more likely to break the item then their estimates (ex: you use your phone at your construction job, and are significantly more likely to break it).
2. It's enough money that it would cause significant problems for me (home insurance, renters insurance, etc).
Otherwise, I figure I am 'self-insuring', and will come out ahead over the long term.