I'll disagree a bit with @TomTom here, but not by much.
First off, if you're even starting to think about a TFS installation, go with 2010. Though there's an upgrade path from 2008 to 2010, it's just not worth the effort you'll need to go through to get there. With 2010, you get a much easier installation, server-side compatibility with more operating system versions (e.g. Windows 7) for single and small-number-of-user installations, even better integration into Visual Studio and other tools.
Now, to address your question as to why not go with the various tools. . . The answer is simple: Integration. It's there with 2008, it's even better with 2010. If you use TFS correctly, you can use it to manage every aspect of your application lifecycle, including requirements, code development, testing, and SDLC reporting. And the great thing is, all aspects of the lifecycle can be linked from one part of the process to another. It becomes really difficult to do that if you're using SVN for your repository and FogBugz for your bug tracking, and spreadsheets for your requirements (etc.).