If anything happens to your memory card alot of data will be lost. people are lazy so if you a very large memory card and can fit more than a days worth of recordings on it then transfering the information and printing it becomes not a rush as it would if you was running out of space.

1) camera gets lost or stolen
2) memory stick get washed (mine still worked)
3) some persons clear memory stick by mistake (some delete/format functions are to easy on some cameras to do by mistake lol)
4) they don't last forever

as always remeber to backup :) I give me mum copies and also take copies of hers having pics in two house holds is a god send (her harddisk gave up so she only lost the ones i did't back up not them all) ;)

http://en.wikipedia.org/wiki/SD_card