Or - a copy button. An example:
Let's say I've created a new seciton, and I want a header template for it. Generally, to be safe, I copy the current header template and work off that. (load the current header template, hit ^N to spanwn a new browser window, do template_new and cut-n-paste the contents of the current template into the new one).
With a saveas/copy button I could load the current, save-as, give it a new name, and go and edit. No cut-n-paste mistakes or mis-entered values/settings. No risk of messing up the current block/template, and I can experiment with the new block until I get it right.