Anyway, having one spelling-checker is better than none. Indeed it is designed to offer helpful hints and suggestions to your writing, not cover or proofread one´s text.
Many criticize, for example, some grammar rules that may leave the green-behind-the-ears writer confused. Namely, creating doubts with sentences where they can be acceptable (passibe voice, etc).
Overall, it can help you make fewer errors, just use it as a tool that help you with (invaluable) suggestions. In other words, it works and the one that uses it have to do his homework as well.