There is no tested safety reason that can be argued as 100% valid to require anything newer then a 2005 rating.
It's total BS and another needless added expense thrown at racers by racing sanctioning marketers. If they are or become required it will be the result of payouts by helmet manufactures. 2016 rating is far superior to 2015 and 2015 gives no better protection then 2005. The only reason for not having to push 2016 helmets on racers is most ever racer could not afford a 2016 rated helmet. Helmets rated at 2016 are far superior then 2005, 2010 and 2015 and the only reason their not pushed is 2016 helmet cost.
From the Snell site and notice there is no impact difference between 2005, 2010 and 2015 except for second impact after a huge hit. If you take a 7.75 hit you ain't going to be around to care about if it can take another 7.75 hit.
"The nominal impact velocities for M2005 work out to 7.75 m/s for the first of two impacts and, for the flat and hemispherical anvils, a second impact of 6.63 m/s. M2010 and M2015 retain 7.75 m/s for the first impact"