My understanding is that prenup only applies to specific assets obtained before marriage, which wasn't much in my case.
Pre-nuptial agreements are contracts between persons intending on being spouses. As a contract, they can vary widely based on whatever terms are contained therein. It's not necessarily only on assets - you can contract out of spousal support obligations, ownership of pets, and many other things...you only can't opt out of Child Support obligations.
I agree, however, that a pre-nupt is not a form of insurance. There is no third party insuring against a certain peril or loss, which is the very effective definition of what an insurer is. Arguing that divorce is a peril/loss that one will be indemnified over between set assets is simply not the same as paying annual premiums to a body that will pay for losses.
I'm not yet married, but we will have a pre-nupt that will be fair to both parties. It will ensure that spousal support cannot be sought on certain conditions, that assets will go where we want them to, etc. It'll be a form of controlling emotions in the event of a marriage breakdown. Anything else will be covered in a will.