Just my personal opinion FWIW.

Some people have reported less than perfect results using the native Windows 7 backup and restore tool. For that reason another free product called Macrium Reflect is often recommended instead of (or in addition to) the Windows 7 tool. I use them both and make full system images once a week.
http://www.sevenforums.com/tutorials/73828-imaging-free-macrium.html
How often one should make a new system image really depends on how much data is changed on the drive being imaged. For some folks once a month may be enough. For others, a new system image every night may be required. So I don't think there's a one-size-fits-all answer.
Again, just my personal opinion about rescue disks. They can come in handy for some folks. Usually those people who don't have a system image and backed up files. Assuming a computer goes belly up because of software issues (a badly corrupted operating system, malware, etc) then a recent system image should be all you need to get up and running again. But if a computer stops working because of hardware issues, then I don't think a system image or a rescue disk will do you any good until the failed hardware is repaired or replaced.
As far as a system repair disk goes, it can be used use to boot your computer to the system recovery options to help recover your Windows 7 installation if you don't have a Windows installation disc, can't find your Windows installation disc, or can't access the recovery options provided by your computer manufacturer. A system repair disc is not able to install Windows 7. In other words, you can use the system repair disk to boot your computer and then access your system image to restore your computer.
Others may disagree with my opinions and will hopefully jump in with some other thoughts.