In the JAMA study they were using joint years as a standard (that's 1 joint per day/yr) and at that level they did not find any evidence that increased exposure to SMOKED cannabis adversely affects pulmonary function. According to the statistics the average user will be using much less than this (around 2-3 times per month). Sounds like they found their comparison in this study though.
My concern will be a lack of education regarding possible adverse effects and folks abusing cannabis, similar to how many abuse alcohol. Instead of having a drink or two, a lot of the younger crowd pushes it to the limit, as I'm sure will happen with MJ. That said, it already happens. But still, the possible negative ramifications do not equate with the weight of the plant's current legal status. It's that simple.
Anyhow, the study for those interested:
http://jama.jamanetwork.com/article.aspx?articleid=1104848