I have seen these used effectively by thoughtful clients. Those are typically ones whom have taken the time to put together a professional request. I have no issues taking time to answer those. However, the vast majority are rather comical. I'll see someone post a sentence or two about a potential job that boils down to "I need help" but says nothing further. Then you get a question that asks something to the effect of what will be the most difficult part of this job. It takes ALL of my reserve not to answer: "Trying to get you to tell me what the heck it is that you want!"
Thank you for the giggle with my coffee this morning. But on a serious note you are right. I have seen a few of those as well I wanted to ask the same thing.
On the other hand.. I have posted jobs with questions, although I do not use the generic questions and I generally write out a very detailed job posting. I use the questions to ask specific questions that are important to me.