As a regular poster on the headphone guidance threads, I'll be the first to admit that I do recommend headphones that I've never heard.But at the same time, I try to avoid giving definitive opinions and encourage people to do some more in-depth research themselves. And while I agree that getting an opinion or recommendation from someone who owns the headphones is ideal, the issue is that that's not always possible. The recommendations thread is one of the first places many people go when they first post on Head-Fi, when they don't really know much about headphones, about the different sound signatures, soundstage, open versus closed, etc. And the problem is, there's too many people looking for advice, and not enough giving it. People post on that thread asking for advice on pretty much every kind of headphone imaginable, and there's very few people on here who will have heard all of them, and most of the people who have don't deign to spend their time in those types of threads. Now this isn't a knock against you more experienced guys, but I do think that this forum needs more people like ljokerl, who are willing to give their time and experience back to the community.
I fully recognize that I'm not even the next best person to ask when it comes to many of these headphones, but I do think that by reading enough reviews and comparisons with headphones that I do own, I am qualified enough to point people in the right direction regarding general categories of headphones (i.e. open v. closed, fun v. neutral, etc.), and to give some examples of popular models that fit their criteria. As much as possible, I try to point them to individual reviews and dedicated threads that can give them more specific details. However, many of the people who need help don't necessarily have the time to do all that research on their own, and need a centralized place where they can ask for advice.
In addition, I don't think that owning a particular headphone makes you an expert in terms of recommending them either. Many early headfiers (myself included), have a tendency to recommend their own headphones in very strong terms to others, partly because that's what they have the most experience with, partly because they haven't heard anything else, and partly because they like to think the made the right purchase. Though I don't agree with everything mbamg has said, I do think that there is a certain degree of value in reading other people's reviews and other people's comparisons to other headphones.
I have more specific answers and things to say, but I lost track of my final point, so I might as well stop here before it gets any longer.
TL;DR: It's possible to be somewhat helpful in recommending headphones you haven't heard, and owning a specific model doesn't make you an expert automatically. Also, we need more experienced people giving advice to the newer ones.
P.S. And yes, I am still in school