I had opened a parcel in front of the courier in the past but that was because there was a little hole all over the package, so I just wanted to make sure if I had to return it with complaint in case something was broken. That was the only time I did and not again until today.
For me to do this... I'd have to actually see the delivery person. I think the people who deliver my packages are ninjas, I NEVER see them. I'll go outside and look, no package, come back five minutes later, boom, there's a package. No sound, nothing. They've also made friends with my dog, apparently, because he doesn't even move when they pull in. The only one I've met/seen, is when my delivery required a signature or an ID. I rarely order stuff like that, so it's next to never that I see them. They did tell me that they give my dog treats, though, so that's probably why he's quiet for them.
I've never bought anything from a seller recommending that I open my package in front of the courier. Having to open the package in front of him/her takes the privacy and fun away from me opening my package. If I was to ever order something from a seller like this, I'd be leery and quite truthfully, I wouldn't buy from them. I shall open my gifts when I want to in the comfort of my home in privacy.
I'm going to guess sellers that request this are probably selling electronics, breakables, or very expensive items. A lot of people pull the "I didn't get what I ordered," or "It was broken when it got here," and expect the seller to fix it. The way Paypal is, which most sellers use, they'll lose their money as soon as you file a complaint, so a lot of sellers are moving toward proof now. Can't say I blame them.