I bought the Lightning to SD Card Camera Reader recently so that I can copy images from my SD cards into my iPad. The main reason for doing that is to act as a backup for my images since I may not want to bring along my laptop especially for short trips. A secondary reason is to be able to view and edit images, although this is not as critical.
There are actually other devices out there that can backup images out field e.g. NEXTO DI ND2901 but they do not come cheap. I also wish to make use of existing devices I have as far as possible. Since I already have an iPad and chances of me bringing it along for trips is high especially when I don't want to bring along a laptop, it makes sense to leverage on the iPad rather than buy something new.
As for getting images into the iPad, there are other ways besides using the card reader such as Wi-Fi. The problem is I shoot RAW, and transferring RAW files over the air is not only agonizingly slow, it gobbles up battery power for lunch too. I would imagine wireless transfer would be useful for scenarios where you want to shoot and have the pictures (preferably JPEG for faster transfer) immediately available on the iPad for you or your client to review. But since my primary objective is just to backup the SD cards, I don't really need the wireless option.
The iPad can import RAW files using the iOS 8 Photos app, but it is slow in displaying the images and this seems to be a bug introduced in iOS 8. Hopefully it will be resolved in the latest iOS 8.1. I just imported 315 NEF files (20.2GB) from a SanDisk Extreme Pro 32GB SDHC UHS-1 to an iPad Air running iOS 8.0.2 and it took around 19 mins and consumed 4% of my iPad's battery.
What about getting the images off the iPad and into the computer? If the iPad is solely used for backup and nothing else, and the original SD cards are untouched, then you can simply ignore the images on the iPad and import directly from the SD cards. Once the import is successful, you can delete the backed up images from the iPad to free up space using utilities like Image Capture on your Mac.
For users of Adobe Lightroom mobile, you may be aware of the auto import feature that will automatically add new images in your camera roll into Lightroom. Unfortunately it will not do so for RAW files so I cannot use that to import the files into Lightroom.
However, the iPad can do more things than simply act as a backup device; you would probably want to view and edit those images you shot with your camera on your iPad. The Photos app in iOS 8 is able to display and "edit" RAW files. I use it to go through the images on my iPad and delete unwanted ones. Thereafter I use Lightroom to copy directly from the iPad rather than the original SD cards. It is probably slower this way but at least I can avoid importing unwanted photos. It's a pity labelling certain images as favourites in Photos will not port over to Lightroom. Perhaps if I import from Photos to iPhoto on my Mac then export from iPhoto to Lightroom, it may work. But it's too much a hassle, honestly speaking.
Last but not least, the card reader only works on iPads and not iPhones. If intended purely as a backup device, the iPhone, being smaller, is obviously more suitable than iPad, assuming same storage capacity. And with the new iPhone 6 Plus, the screen has gotten bigger to facilitate viewing and editing, if necessary. I will almost always walk around with my iPhone but not iPad. iPad Mini is an option, but again, it is not a device I will always walk around with.