In my opinion, the people that are keen on editing want Slashdot to be more of a forum so they can respond quickly without thinking, fix up any egregious errors, and otherwise try and dominate conversations with speed and volume rather than quality. We don't need a lot of "your an idiot", "no you're an idiot". The preview button exists so that you can proof read your work. I think this was a very deliberate decision, and moving away from it would change the nature of the content that you see in these discussions.
Having said that, there are things that could improve (such as enforcing the "no down mod because you disagree rule"), but slow down cowboy, slow down.
No! Not great points!
Editing is another way of saying "revisionism". If you can't edit your post, you also can't claim you "never said that". This goes into building a reputation as well. Some of us are fast and loose with lots of errors, some members you can see always take great care to post perfectly. This tells you something about that person. Don't pander to the anonymous posters here. They don't make the community, the logged in users do. Anonymous posting has a place, but I have no time for people who are unwilling to build a reputation behind a pseudo-handle.
"Don't talk to me about disclaimers! I invented disclaimers!" -- The Censored Hacker