It's not that its a bad place to start to find info and I use it often, but I would never cite it, and tell my students not to either. The fact that most pages can be editted by anyone makes it a risky proposition because the average person is not an authority on any subject. Before the internet, print was the thing, and if your material didn't pass muster, your company got a bad reputation and that would ruin it. What wiki does provide most of the time are citations to PRIMARY sources, which of course are citable, and that is how wiki should be used.
I don't believe the density of PBr3 is 2.28 g/mL because that's what is listed on wiki, but because that's the value on the MSDS they cite.