While it is very important for your translation provider to have the source files, it is also important to consider the environment in which translators work. Translators and editors typically do not work directly in the source file.
In order to take advantage of technologies such as translation memory and repetitions, translators and editors typically work in a CAT (Computer Assisted Translation) tool. While translators do have access to the source files for reference, almost all of them utilize technologies such as a CAT tool to do the actual translation work. Below you can see an example screenshot of what a translator’s screen looks like inside the translation tool.
As you can see in the picture (above), this client is able to take advantage of Translation Memory, and earns discounts on their projects for having TM matches.
When doing update projects, it is also important to consider that the translation team typically will use CAT tools. That said, your translation should run the files against the memory, which should yield memory discounts on an update project, and reduced total cost.