One thing to remember about an insurance policy is it provides two coverages: Indemnity and Defense. Your bond would be the "indemnity." That's the amount that the insurer would pay on your behalf, the "limits." "Defense" is that if you get sued they pay for your lawyer and handle the evaluation and negotiation of the other party's claim. You might be willing to take that on, but you should know that you are doing so.
Also, consider how often, if ever, you leave the state in your car or drive another car in another state. Your policy probably provides coverage in a rental car, and likely even promises to meet the minimum limits if you are in another state. If you travel to Oregon, where the minimum is $25,000 instead of $15,000, your bond might not meet the requirements and you could get hit with a driving uninsured ticket if something happens.
The bond price is the same as about 3 years of insurance. If you put up the bond, you probably have to keep the money in a pretty secure, low earning account. If you could be getting 7% on the money in an investment account, in three years you'd make about $8,000 (I think?) I think that means the $1,400 for the bond is a better deal than putting it up yourself. The bond price is only a better deal if you don't get in an accident that takes the limits in the next 3 years.