Can package can be used as backup?

I think you can, if you have enough time and patience and take care a lot of what you do. But, it's totally not recommended. Packages are suposed to be used to move assets from one place to another (e.g. between two persons or two different projects).

One of the good solutions so far is using Unity's Asset Server (though it has it's problems when multiple persons want to work on the same scene). You can also incorporate different version control systems like SVN. But, both these require you to have a Unity Pro license.

But, again, if you don't have a big project, you might as well save your whole project folder in another location from time to time as a back-up.

You can also find some very good answers about this in another similar question here: http://answers.unity3d.com/questions/1235/version-control-workflow.