I can hardly see any reason why you'd want to defragment your flash. Defragmenting a disk reduces the seek times involved. There isn't anything like a seek time for a flash - it simply doesn't matter in which order its blocks are read/written.
On the other side, the number of allowed writes to a flash memory cell is limited, and defragmenting concentrates your data at lower addresses. But I somehow doubt you'll ever reach a critical number of writes anyway so this probably won't matter much.