The dropbox offer is only valid once per account.
I tried with my s3, note 10.1, and s4 but it only activated once.
The way I got around it is to create 5 dropbox accounts, and activated each to 50gb. (You need to clear app data between activations)
It expires after 2 years, but whatever is on there wont get deleted.
So I have one where I keep my clonezilla images (47gb), another where I keep all my drivers/firmwares (35gb), one where I keep my encrypted Titanium backups for all my devices (41gb), one for my music playlist (24gb) and another where I keep ebooks and audio books (32gb).
I use dropsync from playstore, and configure multiple profiles in titanium backup to manage backups and syncing between all the accounts.
If it aint broke, you haven't modded it enough