the vast majority of misdeamnor prostitution crimes are adjudicated without any offical action taken on the charge as they are often uncontested nolos or continued without a finding in exchange for court costs or similar.

If even ten percent of these actually went before a court we'd need ten times more courthouses.

(still not a lawyer)

Haven't been able to find California conviction rates. But here is how Honolulu breaks it down:

In the 13-year calendar period from 1993 to 2005, there was an average of 423 arrests per year for prostitution.  For the same period of time, there was an average of 223 convictions per year for prostitution.  Conviction rate divided by arrest rate equals about a 53% conviction rate.

That means that 47% of the time, a dismissal, or acquittal, or diversion, or deferred prosecution, or some other disposition other than a conviction was the outcome.  As long as you can avoid a conviction, you can expunge your arrest record and erase all evidence you were ever arrested or embroiled in the legal system.  

Of cases that went to trial, prosecutors had a conviction rate of about 80%.  Of course the cases that went to trial were the ones that the prosecutors worked the hardest on to get a conviction.  Thus, if one takes it all the way to trial, you have a 1 in 5 chance of winning acquittal, or a 4 out of 5 chance of being convicted.  Rather than risk conviction, better to negotiate something that results in an outcome that avoids a conviction.

The moral of the statistics is that prosecutors don't necessarily want a conviction, they just want the defendant to cease illegal activities.