Automatic spell checking tools are great. They're a wonderful technology and have definitely gotten better over time. Without question, they've made life easier for a ton of people and made a lot of students happier with their term papers.
Why, then, is the first thing I do when I install a new word processor turn off the grammar and spell checking?
I'll answer one rhetorical question with another. Why do misspellings of "their" "they're" and "there" really bug some people? Because, like other homonyms, they're all correct spellings according to automated software - which means they don't get caught. While general misspellings have gone down drastically in the last 15 years, homonym misspellings have gone up. Once people started assuming that spell checkers would get all issues in a paper, they stopped paying attention to whether they actually spell correctly.
I'm not going to claim that I spell perfectly, but I do pretty darn well, especially for rough drafts. Typos and misspellings are very rare - and I'll always use an automated spell check tool to go through a draft right before sending it out, just in case I missed something. I'll also admit that I need to use custom dictionaries more to make my life easier. Either way, though, as soon as I start leaning on the crutch that is automatic spell checking and assuming that everything's OK unless there's a squiggly red line underneath it, I'll start losing some of my own ability to catch when I miss a word.
Next time: why grammar checking is just as bad to leave on when you're writing a novel!