Of course all official code should ultimately be stored in source control (real source control, not VSS). However, when creating an automated build on a build server, getting the source code may not be as easy as just doing a single SVN checkout. There may be other steps to effectively get the latest source code:
- Copy in other reusable blocks - Where feasible, you want avoid checking in binaries into source control when those binaries will be constantly updated. Unlike plain-text files, you can't do an effective diff on binaries - it will just look like a mess. So instead of storing just the change set, it will probably need to store the entire physical assembly - which will bloat your source control. So, say you've got a team that is actively working on a set of reusable class libraries, to be shared across multiple departments and product groups. It may make sense to have your product's build copy in those latest reusable blocks (from their build's published output) to some external folder where you store your third-party assemblies, as opposed to constantly storing the latest version in source control. (So, ultimately the reusable blocks are still stored in source control, it's just a different repository).
- Code Generate from input files - You can use code-generation for lots of things, such as SQL base data or your data-access layer. If these files are completely code-generated (i.e. no merge regions), then you may not want to check them into source control, as you'll just face synchronization errors. For example, if you generate your data-access layer, and it's 100% determined from some set of xml files and database schema, then your build server could simply re-generate that code. If you check it into source control, then that version may be out-of-sync with what gets regenerated, and your build fails. In other words, as long as the server can already obtain the code by regenerating it, there's no reason to check it in - and risk checking in something that doesn't match what will be re-generated. (So, the generated files are effectively stored in source control via the inputs necessary to recreate them are being stored in source control).