A normal casting cycle for me (full affliction) is Curse of whatever, Unstable Affliction, corruption, Siphon Life, and Immolate. After they're all on I have about 2-3 shadowbolts I can get in before I have to start reapplying dots. With the improved shadowbolt talent every time it crits all shadow damage taken by the target is increased by 20%. So while other stats are more important to an affliction build, crit is definitely something to look at. If i can get 1 of those 3 shadowbolts to crit all my damage (except Immolate) goes up by 20%.
Hence the importance of crit even in the "all about dots" build.