This is why I use it. I also never erase my SD cards until I'm back home. However, I plan to use the device to backup my files. I've experienced an SD card failure and lost pictures. (Also, you can see the files on the drive through the smartphone app - another great use could be to take the device around to family and friends and share your holiday photos through their ipads etc)
My family go to WDW for at least 14 days but we can only afford to travel from the UK every two years or so. Photographing Disney is a big part of our holiday. My wife also shoots with a DSLR. Last time we ended up with over 140GB worth of files. Last time we took a laptop - not ideal as it wouldn't fit in the hotel safe.
I already had a few portable HD drives lying around. And the fact that my son can use it to stream movies on long car journeys to his ipod touch and keep it charged is a bonus and worth the £30 ($45) on its own
I know that I have come across a bit strong over this but I am just amazed at what it can do for such a cheap price.