A study has been made on the influence of spell checkers on writing quality. The result - a little unsurprising I think - is that using spelling and grammar checking can actually impede performance.
The way this probably works is that you change your work mode from a 'creative' mode to a rules-based fact checking mode. This has two problems: First of all, the spell checking software is far from perfect. Generally speaking the checker will not catch all errors, so the rules you're checking against to see if you're done are incomplete.
Second, you rarely want to be in a rules based mode during writing. Working rules based (with present day technology at least) invariably means that you're working from closed world, severely bounded models of the problem - i.e. the proverbial hammer that nails your english to the floor. A new book that's coming out just now, makes the same point as the experiment only about child creativity and interactive computer games as opposed to old-fashioned creative self-made games.

Writing is more of a search problem in that respect . You're scanning your memory and the situation for appropriate phrases to apply when continuing the text. While I am an AI optimist, computers are unlikely to sensibly support that process in any known near term future.

p.s. I know that classy.dk is a living counterexample to the experiment discussed. Browsers are possibly the worst text entry interface possible.