This year a study has come out showing that reducing the number of people hiking Half Dome has not resulted in a reduced number of accidents:
From Outside Magazine:
Yosemite Permits Aren't Making Half Dome Safer
The logic behind requiring permits on Yosemite's famed Half Dome was sound: smaller crowds equals fewer accidents. In this case, the opposite is true.Here's the study
they reference, same study noted by HL above:
Impact of a Half Dome Cable Permitting Process on Search and Rescue Activity, Hiker Mortality Rates, and Operational Costs Above Little Yosemite Valley
Intro and conclusion from the study:
The summit of Yosemite's Half Dome is reached using cable handrails for the final 146 m (480 ft). Access to these cables was restricted to users with permits in 2010. The authors aim to describe the impact of permitting on search and rescue (SAR) in the region of the park most affected by permitting.
SAR incidents, victims, fatalities, or costs above LYV did not decrease after cable handrail permitting. Parkwide SAR activity decreased during the same intervals. This strongly suggests that overcrowding is not the key factor influencing safety on Half Dome. This discordant trend warrants close observation over 5 to 10 y.