The first "disadvantage" is that you will share your books. That means everything you read your mother will be able to and vice versa. Also keep in mind that if you buy i book both of you want to read turn of the synchronisation cause you kindle will synchronize with your mothers reading locations if not turned off.
The second disadvantage will be that if you at some point want your own account you'll loose all your amazon books. But that also no big thing at the moment because right at the moment it is still possible to remove amazones DRM protection and keep a copy of every book you'll buy manually for the case you move on your "own" account.
I share an account with my wife and i really have no problem just sometimes when we both read the same book, but we found a way to deal with it by deciding who will read first
