I have to keep multiple SD cards with me at all times because all of my gadgets need a different type of SD card. I've had to change my cell phone three times and ended up getting a different brand each time and a different type of SD card for each. My camera and mp3 player require different ones as well. I've got six or seven different SD cards, all with different data in them.
I have to have an SD card holder for them or I would lose them very easily, I'm already pretty disorganized. Even more than losing the data, I would be sad about having to buy more cards because they are pretty expensive, I don't want to keep spending money on that. I've ended up getting a flip case that has different shapes and sizes for different ones. All of my SD cards fit in it even though some are smaller and some are larger. It's a good way to keep it together.
Maybe all brands will one day require the same kind of SD card and we won't have to keep so many with us. But until then, there is going to be an increased demand for SD card holders.