There are probably 20 better ways to use the money and time it would take you to write a business book. You could attend a slew of networking events. You could launch a major advertising campaign. You could start a new business. And so on.
That said, writing a book is not always the wrong thing to do. I know a guy who turns out a small volume in his area of expertise every few years. He uses the books as tools to engage with potential clients. They're his calling card, as well as a way of differentiating himself from his competitors. And it works for him.
I know others who write business books for largely egotistical reasons. They want to establish a legacy. They think they've learned things that will help other people. Some of them even expect their books to find a substantial audience. They're usually disappointed.
Here's my general advice: If your business involves speaking or consulting, you should almost certainly write a book. Being an author does get you respect. If you're a creative person and need a creative outlet, that's another good reason to write a book. My friend and Inc.com columnist Tim Askew is a great example of this and has written eloquently on the subject.
There can be substantial sales and marketing benefits to writing a book as well. Confidence is one. You do better marketing and make more sales if you exude confidence. If writing a book increases your confidence, you'll probably get results. But if you're writing a book for ego gratification and expect it to create sales, you can probably find better ways to spend your time and money.