I think it's ridiculous that there is a restriction to the amount of songs you can download to a device. Most smartphones nowadays have over 60GB of space so it's not like it would fill up the device. I, like most subscribers of Spotify love music but don't have the data allowance to stream. I want lots of choice when out of the house of what to listen to, I'm driving most days but moods change so music needs to change with it. Playlists are brilliant but I'm also from the age of tapes and Cds. I like listening to albums, and just like in those days it's nice to rifle through your albums to choose what to listen to. I think as a family subscriber I pay £14.99 a month for the service. I think it's not to much to ask for unlimited offline downloads.
It has been discussed within the Spotify Team in charge and it has sadly been decided that this idea will not be implemented any time soon. However, Spotify will revisit those idea topics from time to time, so hopefully it will be implemented in the future. ;)