Delphi Build Server – Do I Need to Check in .dres Files?

Well I don't think there is a single definitive answer. But I can tell you what happens at my place of work, and that is that the entire source code tree is under revision control: all my 3rd party components, all the odd C routines that get compiled and linked in, a number of .obj files etc.

It's really important for us to be able to have a repeatable build process and the above is really a pre-requisite for that.

Now, how to achieve that goal with your source control system is another matter altogether, but you should be aiming to be able to check out the entire project and be able to build it with no human intervention.

Just use Python.

I develop in C++ and do my build scripts in Python, and I would find it painful to do build scripts in C++:

• Python makes it trivial to manipulate dictionaries, lists, nested dictionaries of dictionaries of lists, etc. (For example, one of my scripts uses a multi-level hierarchy of all of my tools, tools' versions, and tools' versions' paths.) C++ can do the same with templates and custom classes, but it's much more verbose (which translates to more lines of code, which generally translates to lower productivity).
• Python provides high-level libraries and routines like its XML and JSON handling, subprocess, and os.walk. Again, C++ can do this, but it's a lot more work to find the libraries, learn their APIs, correctly assemble the calls (which are often lower level), etc.
• Build scripts are a non-value-added activity (to borrow a term from lean). It's better to use as high-level a language as possible, to get them done as quickly as possible, to get back to work which benefits your users.
• In my experience, build scripts tend to grow in unforeseen ways. Even if a task seems initially simple for C++, it can get complicated in a hurry. When a new requirement comes up, it's often a lot simpler to tack on handling in a Python script than it is to do it in C++ (which may require finding or reading up on new library APIs, etc.).

Regarding the advantages which you list for C++:

• Adding a single dependency (Python) shouldn't significantly complicate your build. It's already standard on most Linux installations, for example. Thanks to Python's "batteries included" libraries, it may even be easier to manage than the C++ libraries that your build scripts would depend on.
• The type safety that C++ gives is most useful for large projects, not small scripts.
• Python complements C++ very well (high-level versus lower-level, dynamically typed versus statically typed, etc.) and can even integrate with C++ very well (thanks to SWIG and Boost.Python) if you later want to do that, so it's worth learning for a C++ programmer.
• As you said, execution speed should be a nonissue.