My (fairly uneducated!) guess is that people who want to have less than an ounce of marijuana in their home even though it's technically illegal already do so, and that this change wouldn't make more people stoners so much as it would mean that the unlucky small percentage of current casual users who get busted would be less unlucky. This could be wrong of course!

I support legalization, but it's a long way from being politically expedient. Hopefully this minor change will be a palatable and (IMO) positive step.