2 factors going on here. Comfort and control physics.
Carbon rides better then alum bars all other things being equal. This is obvious when riding back to back bikes with both. Risers a typically longer then straight bars, so the longer, the more more harmonic damping ability of a given material(plus of course theres that more leverage thing thru techy stuff thats a big help also, by the way). I have the exact bar in alum and carbon(Answer Pro Taper) cut to the same length and the ride diff is noticeable.
I've tried and tried to "like" a flat bar. Light, cheap, good for climbing(sorta-ish). The pic above was part of that effort yet again. Tried a ct2 carbon flat bar last year and sold it soon after. Just not ergo to me. Its not unlike holding a static bar and trying to control the bike. I feel I have to bend my wrists to fit the bike, and not the bike fitting me. They excel in climbing, ie allowing one to transfer weight to the front more easily, tuck in, accept barends, etc. Too narrow of focus to me. I instantly feel "not part of the bike" again...The riser is what everyone in MX racing(where I came from) has always been using forever. Its simply more ergo and hence I again instantly feel "part of the bike". On technical, this is invaluable. Like night and day for me when I swap back and forth. Setup properly, they go up just as well. I swap parts like some change underwear, so I can tell immediately. My confidnce sores when I have a good riser back on a given bike, especially on the technical stuff.
I tend to cut them down to 24" or so, but thats me.
It at least deserves a try, eh? To see if YOU will like it better.
This is of course all subjective, so I hope this doesn't erupt into a disc vs Vs like argument sorta....again.