From Bryan,

That said, I still think it would be nice if there was either a way to
specify a build target for the "clean" option or have a setting to make
the system default for this option to be "no"; a "force sync to head"
for our Perforce depot using a typical clientspec is ~ 20 GB, and with
that much data, it's simply not an option to delete the entire source
tree as part of a regular build. It also makes any accidental "clean"
builds (using QuickBuild's clean option) pretty costly in terms of time...