Greetings, I'm going to start answering this question now but it won't be a complete answer as I don't have a lot of time on the net. Will return over the weekend to complete it. At some point I will also put up a post under the sub-thread where you gave your personal testimony, regarding my own experiences with ganja and other substances as well.
First, to be absolutely honest, the main reason I continue to use ganja isn't because of any great mystical benefits I might think it has. While I feel it does have some benefits (which I will start to outline below), these were much more significant when I first started using ganja (when the effects were much more dramatic than they are now that I'm "used to it"... although anytime I take a week/couple weeks/month without using it [as I do at least once a year or so] it effects me much more strongly than when I use it on a daily-to-couple-times-a-week basis [which is my general average usage], but still not as strongly as the first few times I smoked). I think ganja helped me access certain areas of my mind that I didn't know how to access fully before I started using it; but that now, I could probably access all those areas without using it, although not as quickly or reliably (at least not without further training of my brain/mind). So the main reason I continue to use ganja is simply that I enjoy it; the same reason animals eat it (or drink fermented fruits etc. for that matter). More than any other substance (except maybe coffee, which I am definitely addicted to) I enjoy not only the feeling I get from it, but also the taste of it, the smell of it... just everything about it. I have also enjoyed getting drunk in my youth, but the drawbacks were extreme and obvious. Even with coffee, I can feel a headache coming on when I drink too much, and I don't like the fact that I seem to need it to wake up properly, no matter how much sleep I might have had the night before. With ganja, any drawbacks seem quite mild and can be avoided by not overdoing it; and I've never felt dependent on ganja to the extent that I need it every day. The times when I go without ganja I don't crave it like how I used to crave cigarettes when I used to be a nicotine addict. It can definitely be a habit, but it is NOT addictive; at least, not in my experience, and at least not in the same way that cigarettes, alcohol or other drugs are.
Now to start with the cost-benefit analysis:
(1) ganja use has facilitated access to what I suppose you could call the non-linear/free-associational aspect of my brain. I say 'facilitated' because undoubtedly that aspect of your brain can be accessed without it; in fact everyone goes there all the time when they "daydream". However, the use of a bit of good ganja at the right time puts one directly into that mindstate pretty much instantly, without taking away too much from one's ability to use the "linear" aspect of your brain as well. (Once you don't use too much). So, for eg., it seems to me that ganja helps me freestyle a bit better (spontaneously rhyming off the top of your head) whilst at the same time I could still hold a "linear" conversation, deal with a phone call where I had to sound "professional" or whatever, etc.
The free-associational flow of thoughts is a bit like what Tracey seemed to be referring to regarding her friend who wasn't finishing his trains of thought, etc., but it seems like he was using too much, too often. To me personally ganja can be a great tool for specific things, like freestyling on stage, not an all-purpose tool for everything. A spanner is great for loosening a screw but it's not the best tool for hammering a nail, for eg. Or, to take another creative activity I engage in as an example, I find a bit of ganja is good for initial brainstorming on an idea for a story or novel, and for the initiation of writing it, i.e., it seems to assist the initial "flow" of writing just like it does rapping/chatting; but it's not so good for ironing out the fine points of the plot or for writing the parts that don't "flow" so spontaneously. If I were to look back on the fiction I've written as an adult (hopefully to be published one day... we'll see), probably in the majority of cases the initial couple of pages were written in a "burst", more likely than not with a bit of herbs in me, whereas the bulk of the pages were written much more slowly, piecemeal and laborioulsy, more likely than not whilst "sober" - though I have written initial bursts without any ganja in me, and have done some fine-honing busy-work type writing with ganja in me.
So, regarding creativity, in my experience getting "high" with ganja assists the unimpeded flow of the initial burst of creativity (drawing on the dreamlike, free-associational aspect of the mind) whilst I'm still "anchored" in the more "lineal" "reality-like" aspects of my brain.
Now what is probably actually happening is that the ganja is not really increasing my ability to free-associate, but rather blocking out the inhibitions and second-guessing and lack of self-confidence that normally would prevent that free association. I know that I can do it without ganja (I'm not paralysed on the mic if I'm out of smoke), but it seems like using ganja kind of taught me how to do it, and it's become a sort of ritual now: and it does seem to give me just that slight extra edge, but of course that could be purely subjective.
Well that's my time on the computer for today. I'll be back over the weekend or next week to continue. I'm going to try to do this in a point-by-point way and try to be exhaustive so bear with me.