I have heard many people complain about plea deals, making it seem like they are just an easy way for prosecutors to take a shortcut. As a former employee of the Salt Lake County District Attorney's office, I know this is not true. Most prosecutors work very hard — often under very limiting budget constraints that defense attorneys in private practice don't face. During Nancy Workman's arbitrary hiring freeze, it was nearly impossible to even replace support staff that quit.
Plea deals might not be desirable, but they are sometimes a necessary evil. They are usually made when the evidence is insufficient or if there is doubt that there will be a conviction. Even if the best prosecutor and defense attorney in town face off, only one will win. Plea deals ensure that a criminal does at least some time, as opposed to no time at all.