But i'll tell you the argument from the police and prosecutor side. I spent some time speaking to a high-level police officer about this some years ago. They aggressively prosecute downloaders of child porn for a number of reasons:
a) No matter how far down the chain somebody is, demand for child porn necessitates child abuse somewhere up the line. Someone is taking these photos to supply a demand. By taking out users you suck up the supply of child porn.
b) Unlike with drugs, which are used by a much larger portion of the population, it is feasible to cut down child porn supply by arresting people on the demand side. Child porn addicts, on average, spend a lot of money and resources on obtaining material, so even the arrest of a small hub can have an impact on demand and break up trading networks.
c) The approach to stopping child porn is to find the publishers and distributors by taking out users. Again unlike the drug market most users of child porn are also distributors - trading is very common. If you find one person in possession you can very quickly map out the network that person is within, which often results in the big and wide busts you hear about on the news
d) This police officer told me that in a majority of cases the child porn user treats the arrest as an intervention. Most know that they have a problem but can't stop and in many of his arrests the child porn users were thankful and became co-operative. It took an arrest to "snap them out of it".
It convinced me (I think - still undecided) that you can't legalize or be soft on child porn possession, as all that demand is going to lead to a child being abused somewhere in order to satisfy it.