Same reason Kienle wore it for Kona, apparently it's only 2 watts slower than the split.
Ben6 wrote:
I found Ventilation always sufficient, though I haven't done any super hot races this year. I did wear it on my first ironman this year, but for my second decided to go with an aero road helmet and sunglasses, purely due to the weight. The Aerohead is one of the heaviest out there, if I am not mistaken it was almost half a kilo in size L (460gr incl shield?), and I had quite some troubles with my neck on the last third of the bike course. Changed to Scott Cadence + Sunglasses, I think its 130gr lighter, and then no problems in the neck, and I also felt like I could start to run better after the bike (tight back and neck of course impacts your run). I do not know how many seconds/minutes this supposedly less aero setup costs me on the bike, but I was happy with it.
Just something additional to consider for a helmet for ironman distance =) I am sure I will continue the Aerohead on half ironmans, but otherwise would love to do some wind tunnel testing to see how exactly the helmet effects my wind resistance