How necessary or important is it?
Important for what? If creating a public persona is useful in the work that you do, then by all means build a web site and devote some time to making it useful and keeping it fresh. A web site could be particularly useful for independent software development contractors, promoting software for sale, and otherwise making yourself and your work as visible as possible.
On the other hand, plenty of smart developers have more work than they can currently manage as it is. Spending time promoting themselves on the Internet is not necessarily time well spent. In fact, the more your customers like your work, the more likely it is that they'll want you to do additional work for them, or to recommend you to others. For such a developer, a web site may not be important at all. It may be an unwelcome distraction, or it might lead to a lot of annoying e-mail from recruiters.
Also, it's important to point out that not having a public web site is not the same as not having a portfolio. Sure, a web site can be a handy way to display your portfolio, but it's not the only way. Here are some others:
- hand someone an iPod touch with a dozen apps that you've written
- send them a message with links to the web sites or mobile apps that you've created
- forward a list of satisfied customers
In summary, having a web site may be an important means to tell the world about yourself. Maintaining a web site is not an important activity in itself, and it's not important to have one if it's not doing something useful for you.