The Battery should last ages. I've had mine since they were released and it's still working.

It can be a hassle using the codes to log in like kirito said. if you log into to the lodestone you'll need it. the forums you'll need it again. billing again etc. some of those arent so bad depending on your internet settings.

But yeah they rotate a 6 digit password every minute or so. but typically any password will work for around 5 or 10 minutes to account for loss of syncronisation between the time on your key and the time on the server.

I'd personally recommend using it if you have it. takes an extra 10 seconds to log into the game but makes it a damn site harder to be hacked and lose all your characters hard work.