The Point of SharePoint
You're using SharePoint because you want to grab people's attention--so the themes that characterize your pages are important. Are you using it to train employees at outlying locations, or educating clients about upgraded products and upcoming events? Maybe you're a project manager, and you want to create a fluid intranet for your staff that lets you share ideas, work on concepts, modify drafts, upload documents, and much more. Each time you put SharePoint to work on a project, you have to give it an impressive, fresh look--something different from your previous or other current projects. Not only will themes make your pages look great, they will also provide an identity for each project you launch.
Keep in mind, by the way, that a master theme is not the same as a master page. A master theme will let you apply color and design plus a header to your site pages. It will not apply to subsites that you create. A master page is a template that is applied to each page of your website, and you can apply your own codes and navigation elements plus a bit more.
One of the problems you’ve run into if you’re trying to customize SharePoint 2.0 is that many products available are for SharePoint 3.0, and you cannot even go there without an upgrade. There are, however, low-cost and free SharePoint 2.0 downloads. Master themes will liven up your work; try some of these sites.