My personal belief on the matter is that something organically grown in the ground that can heal ailments will always be a better alternative to something concocted and created within a laboratory. I especially like Dr. Gupta's statistic of every 19 minutes someone dies from overdose of prescription medication while there is not a single documented case of someone overdosing on marijuana.
Aside from the medical benefits, let's face the honest truth - there is money to be made here. Lots of money and in lots of different ways. The government can make money by taxing and fining improper farming/usage. The government can SAVE money by not incarcerating people for simple marijuana drug use. Citizens can make money by going into the marijuana business - whether as growers or retailers. Citizens can SAVE money by not being forced (or at least having alternatives) to very expensive prescription narcotics.
Now the supposed downsides - the main two that people often cite are (1) addiction and (2) that marijuana is a gateway drug. To the first I'm simply going to reply that - yes, it is possible to get addicted to marijuana HOWEVER - the addiction rates are lower than that of alcohol and cigarettes (both legal substances). To the second, I find this one to be complete balderdash - people aren't going to jump from marijuana to cocaine simply because they've tried marijuana and have gotten a taste of something "exotic and illegal".
If you haven't heard, several TV channels developed programs specifically surrounding the marijuana industry in states where it's legal - I highly suggest you give them a watch. Especially for those people who are against legalization simply because they believe that teenagers want to use legalization as an excuse to "get high".