I can't understand the need for one on a roadbike at all. Like i said, i put one on my mountain bike. I ride mainly on unpaved, harsh, gravel roads and dirt. I've got nearly 15,000 miles on said roads. Oddly enough, the bike seems more comfortable without the suspension seatpost.

Sure it gave but it just proved to me that i wasn't uncomfortable before. The tires are the best place to gain suspension on a hardtail MTB. Lowering them down to 30 lbs on 2.0 tires makes the ride quite plush. Granted you are asking about a roadbike but just wanted to make an illustration. Also it might be worth noting that a standard seatpost can flex more than you might think if you have a lot of it extended.