There is little one can add now to the 'debate' of drug decriminalization: much has already been written on this topic, and for all intents and purposes the issue has been settled. Arguments can go back and forth when the question is addressed in a utilitarian calculus of future good that will come about once drug use is made legal: much too many factors are at play, and no one who claims a certain effect with great certainty should be believed with confidence. It is almost uncontroversial that many incidences of some types of crime will go down, and other socially beneficial effects can be expected nearly immediately, but arguing that their benefit will outweigh some perhaps unavoidable adverse effects is an uphill battle. 

There is an important and a nearly knockdown move* in this debate: ask not "Should we decriminalize drug use?" but "Why should we criminalize drug use?". Appeals to current law can not work as it is question-begging: this is specifically the issue we are addressing. Aside the muddled issue of "what is a drug?" (alcohol, tobacco, coffee, Tylenol, cocaine) and the effects (alcohol being the most violence-inducing) what principled difference is there between the use of alcohol, Tylenol, or cocaine to relieve pain? Anecdotal evidence should not be used, because every time a person is put in jail (or punished otherwise) an important question needs to be answered "Why am I being put in jail?". 

The debate now boils down to rights. This is not an appeal to the 'right to put anything in your body' since if there is a substance that made one go on a murderous rampage for days - there would be clear reasons why one not only ought not take it, but why a society should prohibit its ingestion. The claim of course is that no drug currently used recreationally has anywhere close to such disastrous effects. The question about which drugs should be prohibited from use by the society can only be answered with empirical evidence. Thus, no matter what sort of reason one provides for prohibiting marijuana, morphine, or cocaine, one must answer why alcohol and tobacco and coffee is exempt. 

* Doug Husak provides this line of argument in his "Legalize This" book in a more coherent and eloquent way.