There a reason you keep your backup external.
First, the drive doesn't have to be on all the time. As you may know, Time Machine backs up your drive a pre-set times, so it will tell you to connect/turn on your device.
Secondly its so that the drive not exposed to the same heat/electrical that an internal would be. I have seen it many times before where a surge will take out the hard drive in a computer, but not an external.
Lastly, it makes it possible to take your backup with you. This is important for people with data security/importance issues. They can lock their backup in a safe or take it with them to the office so that its not sitting right next to the original (think fire here).