They actually don't help much, and arguably make things worse, because pilots that have them do things they would otherwise shy away from, like fly over mountains in marginal weather. Cirrus has only had one or maybe a couple more legitimate saves.

Most airplane accidents happen on takeoff/landing or inadvertent terrain collision in bad weather. Neither of which a parachute would help.

Source: Pilot, skydiver aviation nerd.

Philip Greenspun is a pretty accomplished pilot and has some good writings on it: http://philip.greenspun.com/flying/cirrus-sr20

If his coining the 10th rule of programming wasn't enough for me to want to meet him add in the aviation stuff and he's definitely on my top list of people I'd want to meet.