The reason people want to write for other sites is to get traffic and links to their site.
Whether you accept it or not should depend on:
1) Quality of the article - both the information and professionalism. Is this something that YOU would write for your site?
2) Link out to their site: Are they linking out to a place that you would recommend to your readers? Or something that is schlocky that you'd never send people to?
Bottom line, if this enhances your reputation, then go for it. If not, say NO.