This flash card means that I'm good as long as the battery is, and that I can pull video's up on the TF300 quickly after an event occurs.
The head unit in my car will also accept large modules. I prefer not to have to screw with it much, so the bigger, the better. However, if I'm on a road trip I may want to transfer music from an external HDD, or maybe from Amazon. Knowing that I can do it with this little tablet is a bonus.
The 64GB module will end up getting left in the tablet, and I'll mount it to /sdcard. This will be nice for MP3, program data, and general storage. The 128 GB module adds freedom with other electronics.