Ohhh goodness, friends with benefits. Where do I even begin??
I guess I'll just start out by saying I don't really believe in it. But then that's just me and so now I'll explain that it takes a certain person to do FWB. It takes a certain detachment with your partner. Most women are perfectly capable of one night stands but having sex with the same person multiple times without SOMEONE developing feelings is very, very rare. And by FWB, I'm classifying this is having sex with someone MORE than 3 or 4 times.
FWB is so hard because the minute jealousy shows it's ugly head, the relationship is doomed. Both people have to be okay with the fact that neither of them have claim upon the other. They can do whatever they please because FWB is not a monogamous relationship.
However, I understand the appeal of FWB. Not having the drama or the attachment of a relationship appeals to a lot of people (*cough* MEN *cough*) and the convenience of having sex with your partner whenever you want while still having the freedom to go out and flirt and see if something better comes along. There is no pressure to be a good gf/bf. Also, there are people who are reeeally good at sex but who you may not want to hang out with on a regular basis.
But what I would have a problem with is if you are hanging out with your FWB partner AND having sex, why not make it a relationship. You obviously have one of the most essential elements of a real relationship; sexual attraction. Plus, you are actually hanging out with this person and doing things other than having sex so it doesn't make any sense to me when people don't just put the title on it.
The bottom line is that in order for FWB to work, both people have to be on the same page. The expectations have to be clear and both people have to know when to end it in order not to hurt someone. But chances are someone will get hurt, because the majority of the time, having sex with someone many times is bound to develop into an emotional attachment and when that happens, FWB is doomed and is just down right unhealthy. It is rare that I have come across a FWB relationship that didn't end in an ultimatum where one person demanded a title be put on the relationship or the sex would disappear.
I guess it just all depends on your mindset and your value of sex and what it means to you.