While I agree that it's always best to clamp a bike by the seat post, much of this practice comes from the advent of thin-wall steel, aluminum and other "new" frame materials. I have a Cinelli Pro workstand (can't remember the model number) that I've used for about the last 15+ years, and it's still one of my favorite stands. It uses a quick-screw clamp arrangement that makes it very easy to moderate the clamping pressure EVERY TIME you tighten the clamp, unlike the Park clamp which needs to be adjusted BEFORE you flip the clamp lever. Indeed, the Cinelli was designed to clamp onto the frame tubes and has slots in the plastic jaws to allow clearance for the top-tube brake cable.
Even though I now have a Park Pro, double-headed workstand that I use for most of my major work (and never clamp to a frame with it), I still use the Cinelli, and still occasionally clamp (lightly) on the seat tubes of vintage frames. I've been doing this for years and have never damaged a frame or even a decal, because I know where and how tight to clamp. Another trick I use is to cut off the top part of cheap, white athletic socks, double fold them, and slip them over the clamp jaws. Perfect protective little "mittens". The advantage of clamping a bike by the seat tube, is that it gets the drive train right up there in your face. That's why I still use the Cinelli for quick inspections and adjustments, or just to display a bike.
Park also sells a device that is designed to insert into the seat tube (in place of the seat post), has a wedge-type screw-clamp arrangement, and a second post parallel to the seat tube to clamp in the workstand. A handy device that allows you to "virtually" clamp in the seat tube area. I also have an Ultimate portable work stand that I use for race support and swap meets. I like the screw-clamp mechanism on the Ultimate, and the fact that it's nice and tall.
Wayne Bingham
FC VA