Er... if you're serious about writing, you should be training yourself to write correctly the first time round. It's a bit like cooking. You might hear people argue it doesn't matter what a plate of food looks like, as long as it tastes good. Certainly a delicious looking meal which tastes disgusting is completely wrong, in the same way as a perfectly written novel with no plot or character development is wrong. But the presentation of food is important since how people react to taste is also dependent on sight and smell, plus if the meal looks a mess, people will make the asumption that it tastes awful so won't try it. And if it's a mess in the pan, it's very, very hard to make it look good on the plate.
There will be mistakes in first drafts (and third and fourth, as Ursa points out), and yes it's important to write that first draft quickly to get plot and ideas down. But it is quicker to put in proper punctuation as you go rather than have to spend laborious hours going round afterwards correcting all of it.
As others have pointed out, posts on line are different -- but again there is a but... There's a proverb: he who takes no care of little things, will take no care of great ones. If a person takes no care at all in such posts, it does give the impression she/he is unlikely to care about her/his writing as a whole. And Ursa may be more relaxed about his posts, but that doesn't stop him writing them properly, editing them when he sees mistakes, or feeling irritated with himself when he discovers errors too late for editing.
If you get into the habit of writing clearly and reading your stuff back, it will be of real help when you write more important things than posts here.
