I don't back up my OS, seems to be a waste of space to me. I rather back up all my data and games with the little 2TB backup drive that I have.
That may work for you. But as a general rule it is not viable. If you have no images, the option in case of a major problem is a reinstall of the system, all updates since the dawn of time, all programs (appr. 50 in my case) all settings have to be redone, etc. Last time I did that, it took 3 days elapsed - mainly because the downloads take forever. E.g. WAIK alone takes over 1 hour (1.7GBs). Similar with big updates like SPs for the system and Office.