I don't know about the whole moderation point-Studies have shown that even very light drinking shrinks the overall brain size somewhat. I'm sure marijuana has some sort of effect too, even with just moderate use.
Why would you hurt your brain for fun? It's your brain. It's who you are, and for most of us on this forum-it's from it that determines how successful we are/will be. Go do something else enjoyable.
That aside, I've seen some people get pretty screwed up due to marijuana use. One that's been doing it for 35 years or so- I'm sure he did dabble back in the 70s with harder drugs, but I know the absolute majority of what he's done has been MJ. You can see the lag in his brain whenever he thinks of something. It's like there's..latency.
And of course, there's examples of people my age who have done it for a few years or more, and you can just see them deteriorate mentally. All for a high.