Why am I doing this...
The statement "weed isn't as bad as alcohol" is not a good argument. That's only a statement about weed relative to alcohol, not about weed by itself. Weed could be bad, and the fact that there are worse drugs does not make it any less bad. As such, the argument can be dismissed.
As for whether or not weed will be harmful if it is societally accepted, we have almost no information. Using countries like the Netherlands where it is already legalized as proof is faulty because there are a number of other factors that need to be considered (ethnic homogeneity, preexisting crime rate, cultural approach to consumption (French view of alcohol vs. American view, for example), income disparity, etc).
In regards to the "you can't talk about it if you haven't done it," let's take an extreme example: meth ( I am not comparing meth directly to marijuana, I'm only using it meth to demonstrate a point). I've never done it but that doesn't change the value of my beliefs about it. I can still reasonably determine what affect it has on its users.
The "it's a plant" argument is faulty. We are not concerned about its origins, we are concerned about its affects.
The "people already use it and it seems ok" argument is also faulty. Legalization increases access to the substance and that's what we are concerned with. There's no reason to assume that because it is already used (mostly) illegally that it will act similarly when legal.
In regards to legalization, I am of the opinion that mass-adoption of legalization laws is moving too quickly. We don't know how this can affect Americans over time. The best approach would be to wait and collect data over a generation or two on how legalization has affected the populaces of those few places in America where it is already legal. If there's no distinct changes, then we have reason to proceed. If not, end the experiment.
Having said that, I do support lowering the penalties for possession. We all know this disproportionally affects african-Americans, and it's not out of the question that lowering the penalty down to a fine may benefit the community in some way.