Originally posted by PDL Copying and erasing will leave fragments of files laying around (due to the nature of FAT file systems) which can cause issues.
Generally correct, but not entirely in given context
Fragmentation occurs, when you delete pieces files from inbetween other files, leaving a hole AND when trying to create or copy there a new file which fit the gap entirely (being too big). In that case file is split and different parts are written into different free holes (clusters). Filesystem can handle that, it is not a problem in itself, the problem is that it becomes slower and slower to both read and write files when each file has to be split or glued into one again.
It should not present a big problem for memory card where access time does not really depend on the location of the pieces and besides, the data is usually deleted in bulk, thus creating a lot of empty space in one go, not variable size empty holes. Therefore there is no real need to worry about fragmentation in camera memory cards.
As for aforementioned link, then knowing how filesystem works in general I can't but to disagree with all due respect with following: "A contributing factor to card failure can be the existence of previously deleted photos." Think of your memory card as your USB-stick (which it essentially is). Do you reformat it every time you download stuff from it to your computer? Have you ever heard that it may fail because of previously deleted files? It'd be nonsense. Think about it, we used to run computers off the FAT filesystems and 99% of the failures were result of power loss, not the filesystem itself.
Filesystem is probably the most robust and time-tested component of memory card. If there are problems then because of interrupted or interferred operations, physical failures etc. If card fails once and reformat seems to cure it, it may have been a glitch. If card fails more than once and every time format seems to cure it for time being, it's time to replace the card or check the camera, because something else is fishy there, not filesystem.
Generally the only rational reason to prefer formatting to deleting would be the speed. But of course, its better to let the camera do the formatting with suitable defaults rather than Windows.
Sorry for long post, it's just that that "formatting is better than deleting" thing has been bugging me for ages.