Matt Laur , May 27, 2008; 08:47 p.m.
There are a lot of things that can go wrong when the card has been touched (especially, written-to) by an external device. Even just popping the card into a card reader on your PC, and having its native thumbnailing routine plow through it trying to help you view that device's contents... that changes the data on the SD card. It's up to the device doing the writing (your computer, a storage device, or the audio recording widget) to honor the file system on the card, and stay within the bounds of the formatting found thereon... but they don't, always, or might be interrupted by some other unforeseen event.
More to the point, it's not unheard of to pluck the card out of the other device (a card reader, for example) while the thing that's talking to it isn't quite done closing up the files it has just altered. The very nature of the file system on the card (or a hard drive, for that matter) is that writing a file to it means pushing that data over AND then updating the file system information that describes the name, date, etc., and other housekeeping data that makes the file appear as in item in the file system. If any of that becomes half-baked, the file system is then corrupt. Reformatting it clears the deck, and creates a nice, fresh file system and housekeeping space on the storage device/card/drive.
Because the last thing you want to go wrong is the process of the camera writing data to the drive and correctly establishing the file name, etc., it's ideal to let the camera set up shop on the card in the way that it sees fit. Formatting the card also gets rid of any other unpleasantness that might have found its way onto the card... viruses, hidden files, etc.
MUST one, on any particular schedule? No. But when I care about what I'm shooting, and the card has been updated in any way by another device, I take mere seconds to format the card in-camera. Every time (well, every time that I care about the images or the event I'm shooting).