There is no guarantee this works every time. Virus definitions may change e.g. some value in some of avast! INI files (it certainly happens) - and when you restore this "backed up" definitions, your program will be in an inconsistent state and strange things may happen.
Generally, manually copying any files into avast! folder (such as this "method") is not recommended, it may break up things. Besides, I don't see any reason to do that - if you want to store a particular version of virus definitions, you can download the vpsupd.exe file.