While this practice is fairly rare, and I have never employed it, I fail to see the problem:
1. Not posting URL can protect the domain from trademark crawlers and search engines, which helps the buyer and seller retain traffic and domain value.
2. It is the sellers right to disclose this type of information to whom they see fit.
3. Asking for a PM to provide valuable informaiton helps to ensure that the buyer is serious enough (how long does it take to request the URL via PM, really?)
4. Threepwood has done a good job of describing the traffic, it's source, value, and type. If this is not enough to at least provide a preliminary basis for interest, then you are obviously not a candidate for purchasing the domain.
5. I could find nothing in the stickies or forum rules that prohibits asking serious buyers to contact them for the domain, and it is a practice used not infrequently on this board. Am I the only one that resents new rules being imposed without notification? Not that it's all that inconvenient -- it just feels overbearing, and creates an autocratic atmosphere. Just my $0.02.