Marijuana Possession and Prison

I did a little digging, trying to find data on the number of persons actually in prison for marijuana possession (since this is a favorite claim of those arguing for legalization--freeing up prison space).  The most recent data that I can find is from a 1997 survey:  http://www.bjs.gov/content/pub/pdf/satsfp97.pdf  

While 27.1% of state prisoners and 5.3% of federal prisoners were there for drug possession, most of those aren't there for marijuana.  For all drug offenses (including trafficking), 12.9% of state drug offense prisoners and 18.9% of federal drug offense prisoners are there because of marijuana. (p. 2)  Even if possession was punished as severely as trafficking (which it isn't), that would be only 3.5% of state prisoners and 1% of federal prisoners are there for marijuana possession.

Here's some more recent data, cited as coming from Columbia University's National Center on Addiction & Substance Abuse:

It is exceedingly rare to be incarcerated in the U.S. for the use or possession of marijuana. According to the National Center on Addiction & Substance Abuse at Columbia University (CASA, 2010), less than 1 percent (0.9%) of jail and prison inmates in the U.S. were incarcerated for marijuana possession as their sole offense.  Excluding jail detainees who may be held pending booking or release on bond, the rates are even lower.  Prison inmates sentenced for marijuana possession account for 0.7 percent of state prisoners and 0.8 percent of federal prisoners (see Table). And, considering that many of those prisoners pled down from more serious charges, the true incarceration rate for marijuana possession can only be described as negligible. 

There might be a case that repealing all drug laws would make a real difference in our prison populations.  But that means legalizing not just heroin and cocaine, but also meth, LSD, and toluene abuse.  That's not such an easy sell as legalizing marijuana.  At that point, it ceases to be an principled libertarian argument.