The arguments for and against can go on and on with both side bringing up some very valid points. It isn't the experienced skier that needs the helmet as much as the SPORE's (Stupid People On Rental Equipment) IMHO.
I'm on Ski Patrol and we process 1,000+ accidents/year and I would estimate that 75%+ are beginners that do not wear helmets.
This isn't just about the individual that is injured. The EMS infrastructure can get overloaded by an overabundance of "critical" injuries. If a helmet can reduce the numbers, all the better.
Cost of helmet, $50, cost of a brain?