Cooperation is an important  factor in many white collar cases.  It allows the government to secure important information needed to build a case. 

Clearly there are costs in acquiring that information.  One cost is the lower sentence being offered to the cooperator.  A second cost is that when the government offers a deal, they are also placing the reliability of the evidence into question when it  is presented to a jury.  After all, someone pleading to a lesser charge is doing so to acquire a personal benefit and if that benefit is significant, the testimony can be questioned.  Jurors may ask: Was the individual motivated by the opportunity to receive to lesser sentence, and are they telling the truth or merely lying in order to obtain the benefits of the plea deal.

But without cooperation many cases would not exist.

According to the Wall Street Jrl (here) the government has just made a deal with a former chief financial officer of Qwest Communications International Inc.  This individual will plead guilty "to one criminal count of insider trading" and in return there is an agreement to cooperate with the government in their investigation.   This is likely to be a significant advancement in the government’s investigation as this is the "former chief financial officer" and "is the highest-ranking former Qwest official to be charged with a crime related to accounting problems at the Denver phone company."

(esp)