Pure Assigned Tasks (ATs) are very unfavorable to the lower performing gliders in the contest, even when taking into account the handicaps. If pilot A takes 3 hrs to complete the task, and the same proficiency pilot B in a lower performing ship takes 3.5 hrs, Pilot A will usually beat B.

Pilot A spends a higher percentage of his flight flying during peak conditions of the day.
Pilot A has more flexibility as to when to start
Pilot A has more time/opportunity for a re-start if desired

TATs are much better in that each pilot can complete the task in the same minimum time (if the TAT is called properly) and therefor get the same window of opportunity with the weather that day. At the same time, they both have the opportunity to show more skills in choosing the best energy line, or risk extending the flight to take advantage of a particularly good cloud street, etc. This is especially important with mountain flying.

Perhaps that is why they are so rarely used by CDs.

Matt