They offer you a huge amount of space knowing that most users won't ever actually use that much for their website. However if you're using it as backup space for your music, documents, photos, video etc... then you will.

Basically their servers don't have enough capacity to store every user at max usage. Just shoving more disks into the servers isn't all they have to do. They need to make sure their backup system can cope with the extra data. Their compromise for this new 50GB backup service is that its on another server and that server isn't backed up at all.

Not sure whether I'd trust this to be honest. I know they say its meant to be a backup and not your only copy. I'm just curious as to how reliable this server will be. It could be a ancient server wedged full of drives in RAID0 for all we know...