Most of the things tooltool downloads are large. They end up copied in triplicate on the local host: in the local tooltool cache, in the current directory, and then unpacked. I'd argue that after the unpacking is done, the artifact itself is no longer required and the client should delete it. From what I can see of TT use in the build process, this wouldn't cause a problem, but I'd like some confirmation of that before i go breaking things.
I don't think the build system cares where the files come from, it just wants the unpacked result.