Don’t get me wrong. Spellcheckers have bailed me out of potentially damaging situations many times, and I doubt I’m the only one.
Unfortunately, many amateur writers (even some professional ones, too, I’m certain) use it as a crutch, assuming if there is no red, squiggly line, then the writing is perfect.
Consider this paragraph, however:
Once, eye looked out my window and saw a brown bird sitting in the tree buy the quite street. It chirped softly a gentle song that soothed my weary mined. Suddenly, it stopped, then leapt two the grass below. It hopped around, looking, I presume, for a tasty bight to eat.
A spellchecker would find no misspelled words in this example despite the fact that there are several words that while spelled correctly are used incorrectly. (How many can you spot?)
Hiring an editor—or at the very least getting a second pair of eyes to look over your copy—is one way to avoid embarrassing prose like that above.