Let's assume laws under legalization would make pot similar to alcohol (21 age limit, no driving under the influence, etc.)
1. Other Substances
Alcohol impairs you more, yet is legal. Cigarettes offer all of the same health concerns but offer no significant benefit (high) and most cigarette smokers smoke more than marijuana users on a daily basis. Also, marijuana has been medically proven to be non-habit-forming, unlike cigarettes and alcohol.
2. Tax Revenue
The government could tax and regulate the marijuana industry, generating state revenue at a time when it is needed. Also, the birth of a high demand industry would foster jobs and economic growth.
With the ever-growing popularity of marijuana, middle-class people who are not drug or street savvy are having to deal with dangerous people and frequent the more suspect areas of town to procure their product. Legalization would stop this. Also, legalization takes away a large portion of criminal funds, without the cost of policing. By offering the legal sale of marijuana, the criminal organizations who deal it now are out that revenue.
The medical purposes of marijuana (while they may be suspect) would be improved with improved quality. If people are able to grow it freely, it can be privatized professionally and developed at lower cost and higher quality yield.
5. Civil Liberties
The political history of marijuana is suspect. Many believe it was outlawed under false pretences. Also, in a free country, a citizen should be able to indulge/harm themselves in any way they see fit as long as they do not infringe upon or danger the rights of others. While other harder substances may offer this risk, the private consumption of marijuana doesn't.
Post objections and opinions below, I'll be sure to answer them