I guess it would just depend on the situation. I can't imagine needing one for a 1st time marriage where you both come into it with basically somewhat equal footing.
I know we have run the gamut in our marriage, so we have pretty much covered every base except a divorce. LOL! At least financially....neither of us have won the lottery or a big inheritance or anything like that though. We started out the marriage wtih me making a lot more than him, eventually him making a lot more than me (along with switching around a couple of times), it's a running joke because his car is in my name and my car is in his name due to when we bought them (right now I'm a SAHM so have no income, hence the reason my car is in his name) but we have everything co-mingled. I take care of all the bills so it's all basically ours.
Now, if I were coming into a 2nd marriage and had some specific property or reason (i.e. specific trust funds for the kids or something) that I wanted to make sure was for the kids, then I could see where it might be a good idea. I don't really know since things ebb & flow so much in the marriage I would have to think about just exactly what I was putting in it and why it was important.