The good thing about Sasige is: You never need to update to a new version. If the version, that you currently use, is perfect for you, you don't need to bother with upgrading and maybe broken support after that because of incompatible changes.
No security patches required
Because sasige is a tool the generate static files, there is no security problem here. There is no dynamic script language that is required when the website has been generated. Hell, you can even open it directly from your filesystem in your browser and it works.
Keep in mind that this assume that you DON'T use the built in debug server for production purposes.
Upgrading Sasige, when it's useful?
Obviously when you run into bugs that already have been fixed. Also when you follow Sasige and you see a new feature that you want to use.
Upgrading a theme, when it's useful?
Same here, when you run into bugs that already have been fixed.