I don't think they are horrible and I don't know how I'd feel about signing one. I'd only do it if I actually had valuable items or lots of money that I'd like to keep if the worst should happen.
In today's society, you are more than likely to get divorced, sadly. Why not plan for the worst and sign one? I can understand that people would take it wrong and think their spouse is betting that their marriage would fail, but people and circumstances do change over time. Better to be safe, then be sorry.