Thought I'd respond as I'm a writer as well, who's just finished a novel (minus editing and revision). About half of it was written "under me sensi" and about half was written "straight" (well, all of it was written under the influence of coffee which I am totally addicted to and find it hard to wake up properly without, no matter how much sleep I might have gotten the night before.)
I see what you are saying about the tangential thinking that ganja can encourage. To me it's great for brainstorming ideas and starting out a story when I don't know where the story's going yet, not so great for developing the story in a pre-planned way or for working on the little details and smoothing out the kinks. Although there were parts of the pre-planned writing that I did under sess, and parts of the brainstorming that I did "straight". Sometimes it seems to me that I write better, more fluidly, and the process of writing is more enjoyable when I have a *small* amount of ganja in my system. But looking back over the whole thing now, I can't detect any difference at all in terms of quality [admittedly subjective] between the parts I wrote "high" and the parts I wrote "sober".
The inability (or unwillingness?) to see things through to their conclusion is I think a common problem for many writers regardless of what substances they do or don't use. (Not saying that ganja couldn't exacerbate it). For some, ceasing to use herb (at least while writing) could be a solution for this. But regardless of the use or non-use of any substance, I think the real key to completing writing projects is focus, discipline and enough confidence to balance out our (very necessary) self-critical tendencies. I used to never be able to finish stuff cos I'd be second-guessing myself all the time, thinking "ah that's not good enough, the hell with it", thinking "ah that's not a totally original idea after all" (what idea is *totally* original? Haven't heard one yet), basically thinking that if it ain't a masterpiece on a "Native Son"/"Moby Dick" level then it ain't worth writing. Basically I just set myself targets like writing a certain amount every day and met those targets when at all possible, and concentrated on writing what I like to write how I like to write it, rather than trying to be the next Richard Wright.
At the end of the day I don't think it made much difference if any to the quality of the book, whether or not I was "high" when writing parts of it. Probably the idea that ganja makes me write better is an illusion, but I don't think it makes me write any worse.
Where it really feels like ganja use *does* have a beneficial effect on my creativity, is in music. Ganja just helps me get into music more, lose myself in it til I become a part of it and the part I need to sing or chat or chant just comes to me "automatically" as it were. There's a reason why blunts are often passed around a freestyle cipher, it just helps unleash that flow! I can spit my written rhymes equally well with or without ganja, but as far as freestyling is concerned, it seems like I'm always better when I got a *little* ganja in me (again moderation is the key... too much and I'll get tongue-tied or spit nonsense). It's that brainstorming/tangential thinking aspect, and that's what off-the-top rhyming is all about. Ganja also eliminates "stage fright" and boosts my confidence when performing, to me. I know it's a LOT of musicians from Louis Armstrong to the Beatles to Bob and Peter to Fela Kuti, etc. etc. who vouch for ganja as a creative aid specifically for MUSIC.
Then again it could be that relying too much on ganja for musical inspiration could lead to your innate musicality atrophying. I could see that argument, and I can also see Ras Marcus' argument as far as putting oneself at risk of imprisonment when ganja is not absolutely necessary (and I don't think it's *absolutely necessary* for anything). That's a bit less of a concern here in britain though, where the worst they can do if they catch me with a personal stash of ganja, is issue me a "caution" and confiscate my bud so the damn cops can smoke it up themselves. Unless you're dealing or growing or are an ex-con (for whom the penalties are much harsher for any infraction of the law), you ain't going to prison for ganja in Britain. They have to issue you with three cautions before they can even charge you a fine.
Well with that said, I ain't tryina argue with those who say not to use ganja, if you feel that ganja is negatively effecting you then you should stop using it. Unlike other more addictive drugs that's relatively easy to do. I certainly have no withdrawal symptoms when I go a month and don't use herb. So if you feel it's effecting you negatively, then stop using it. Right now though I personally feel it still has benefits for me, benefits which are best experienced through moderate use and which diminish rapidly the more you use it. (Cos if you smoking/eating/drinking ganja all day every day like how some people do, you're not getting "high" at all anyway, you're just maintaining a sort of dull buzz that you've deluded yourself into thinking helps you cope with the world.)
Ultimately to each their own, not everything's gonna effect everyone the same way. As for Eja's question: anyone who wants weed nowadays can get it, so it wouldn't make hardly any difference were it to be decriminalized and allowed to grow everywhere. Perhaps more people might experiment with it initially, but I think the level of use would level out to about the same as it is today. Ultimately in my opinion you have to allow every adult [let's assume we can agree on a reasonable definition of "adult"] freedom of choice, including the freedom to do things that are potentially harmful to them as long as they ain't harmful to others who have no choice in the matter. Of course you could argue about what precisely consitutes "harmful to others"- does a parent have a "right" to drink or drug themselves into a stupor when they're supposed to be looking after their youths? I would say not...
At any rate if you look at Holland where ganja is legal (or close enough to legal that it might as well be), Holland doesn't have a higher rate of ganja use than other parts of Europe where ganja is illegal. It has a lower rate of ganja use. (And the same applies to other far worse drugs which are decriminalized in Holland- Holland's had about the same number of junkies for ages, they don't have to rob people to get their smack, they're not contracting and passing along AIDS from dirty needles, and since they're not as stigmatized by society they probably find it easier to seek help and get off the drugs. Whereas in countries like the U.K. where drugs are criminalized, there are more junkies every year, who rob and murder for their fix, not to mention all the gangland violence that results from drugs' illegality.)
Well that's just 2 more cents on the subject. P.S. I haven't had any ganja today so any grammatical errors and awkward sentences in this post aren't as a result of that.