This is how it works - you have files on your drive, you remove one somewhere down the line - a gap between files is left. Then, you put on a bigger file and since it cannot fit in the hole, it divides itself into two parts - that's fragmentation. Gradually you get more gaps and files become more fragmented, and since utilities that can defragment WBFS are not user-friendly, it's a pain in the ass. Eventally you end up with an under-performing drive, or in case of WBFS, a corrupt one.From what I understood, there is no need to worry about fragmentation..
Look, I used WBFS when I innitially bought the Wii and switched as soon as it was possible after two instances of corrupt data. WBFS is crap and I know it from personal experience. Use FAT32 and occasionally defragment after file operations.