Here's my take on them, as uneducated as it is and non-fact based.
First off, I voted no. Now, I am thinking of the age group of my DD who is 13 and is just now trying to "find her way" with boys. I may have a VERY different opinion when she is a senior in high school.
My problem with this is the "familiarity" concept. I think there are some good things with boys and girls being "together" so much these days. They learn to relate better between male and females and I also think it is healthy for males and females to truly have a friendly relationship. I don't think this really existed in the 70's/80's when I growing up--although it was starting.
I think, though, that the familiarity has made it easier for the "casual sex" explosion that seems to be going on with middle and high-schoolers. You know, the whole friends-with-benefits thing. It is really sad to see that the majority of kids don't truly "date" or "romance" each other anymore. They just meet somewhere and hook up. I realize this is a generalization and I know there are exceptions to everything. But, I think that the co-ed sleepovers are just another way to take the "mystery" out of the male/female relationship.
I honestly do NOT think any sex is going on at most of these parties and that is not my reason for saying no.