The gist: whenever someone is about to be put in jail for any reason, they ought to be given a reasonable answer to the question "Why am I being put in jail?" There simply is no satisfactory answer for current drug charges.
Bad answers to this question would require, out of consistency, criminalization of alcohol, tobacco, skiing (dangerous!), pizza (bad for your health!), etc.
I highly recommend the book Legalize This!: The Case for Decriminalizing Drugs (Practical Ethics Series) by Doug Husak