[Yo-Yo]

"I continue to get suggestions that the third-stage spin is a 'feature,' not a malfunction, and may be associated with guidance, or decoy deploy, or enhancing hardness against U.S. boost-phase antimissile weapons," he wrote in an e-mail.

My guess as well, to counter laser based boost phase weapons under development.

[DesertRhino]

Obama cancelled that airborne laser, didnt he?

[CurlyDave]

Spinning a missile is an obvious and inexpensive method of degrading the performance of a laser weapon.

The makers of laser weapons have countermeasures which re-increase the effectiveness of the laser. Sometimes not as much as without spin, sometimes more effective.