My .02 worth.

I am a 200 pound rider and very fit at that weight. Just a big guy. I swear by my carbon post on my Aluminum Felt F-75. It has a carbon fork and carbon seat stays too. I first replaced the stock Al with an Easton EC70 but it cracked after a about a thousand miles.

Now I am running an Alpha Q carbon seatpost and have over 6000 miles on it with no problems.
I do think it helps dampen vibration on those long rides. I do lots of Centuries, sometimes 3 in a month and sometimes I throw in a Double Century.