While building (e.g. world), one certainly cannot switch the branch. And building tends to take a while.And as for git usage: You can create as many clones as you like, but why would you, just for looking at a different branch?
And typically, while I glance at the output of the test build, I suddenly remember those security patches for the production version which I wanted to apply. Or something similar.
(I do already run the scripted builds from zfs snapshot/clone, for that reason.)