1. Well yeah it's different. Love for a lover comes with territorial feelings. Sharing a lover can bring up feelings of jealousy, anxiety, and a whole lot of other negative feelings for some people. It's not for everyone, but it is possible to have feelings for more than one lover. Our hearts can be very big, even though it can cause a lot of heartache.
2. Same reason as anyone wants marriage - status, stability, continuous emotional support and love, financial security, health benefits, etc. it's not just wanting someone to physically care for them when older. These people (polyamorous) want to have their cake and eat it too. Some people can manage marriage and dating successfully. Many people cannot.
3. And I agree. The couple in the video don't appear to be in a healthy situation. I feel for their child having to be in that mess. She should have cut husband out if he couldn't agree to monogamy. It's obvious she is a bit insecure with the whole thing. Being in an open marriage out of duress definitely will shorten her lifespan.