When you get married, you should "put your money where your mouth is", so to speak. God intends marriage to last a lifetime, so going into it with anything less is wrong.
using that "plan only for the best" logic, there would be no need for seatbelts, saftey nets, vitamin supplements or even insurance. you see, i drive/live/eat healthy/carefully, exercise regularly, don't take unnecessary risks, and try to use good judgment.
in many cases, we do not take safety precautions not for pessimistic reasons or insecurity, rather, we do it because we cannot speak for, or control the actions of another person. sometimes unavoidable circumstances happen. at the end of the day, i can only be accountable for, and speak for myself.
why would anyone think that ONLY planning for the best case scenario would be an exercise of trust and virtue? a big of part of life (and maturity) is being reasonable about what can (statistically) happen, learning from others' lives, and planning for multiple outcomes.
also, i would like to believe that the man who would marry me would want me to feel secure, and love me enough to want to provide/plan for my future (and the reverse is true as well) in any outcome.
for those who think that love is diminished by a pre-nup, i'd ask, isn't real love more than "i love you (only) as long as you're married to me"?