You can be sure now that with the growth of Firefox and now the launch of Google Chrome that more and more visitors to your website will be using something other than Internet Explorer.
It is important to check as sometimes different browsers will result in code displaying differently. For example on Rainbow Tourism there is a minor difference on how Internet Explorer displays our site versus Google Chrome and even a difference between Firefox 2.0 and Firefox 3.0
You can always find out how your visitors check your web statistic reports. Our own statistics show that 68% of our visitors currently use internet explorer, 22% firefox, 8% safari, 1% opera and we have other browsers listed.
I suspect in a month or two that google chrome will have gained a small share at the expense of both firefox and internet explorer but it will also mean people will be more willing to switch browsers.
So why do I need to have more browsers on my desktop - the key reason is to check that your website functions as you expect it to - if it isn't get your webdesigner to fix and make your website code friendly to all browsers.