I've thought a little more about this issue. Slashdot at it's best is much like formal debating. A topic of discussion may arise and one member will lay out an argument; sometimes appealing to logical reason, sometimes to emotion. Sometimes the argument will be made with references to lend more weight, sometimes not. In the good case, someone with a differing view will come along and present their own argument, trying to be more persuasive in the alternate view. Often the two submitters will cross-examine each others arguments testing the weaknesses. If the moderation is doing its job, and both submitters are worthy, then all of this debate gets moderated up to high insightful scores, and the casual reader gets to see two well argued positions. What might not be obvious here is that there are two very important things that "no editing" provides. The first is that the arguments are presented as is, there can be no revision. Weaknesses in the argument must be addressed by subsequent posts. This is important as the reader can see the point - counter point - counter counter point, as it occurred. The second is that the type, preview, submit, "it'd better be right or I'll look like a goose!" cycle encourages people to take their time. Good insightful posts often take 1/2 hour or more to prepare. It's like a mini journal, reviewed by your peers.
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.