Click the link 2 posts above. It gives a fairly good explanation fo the type of fragmentation that occurs. You are right in that SSDs do not suffer from the standard block fragmentation which occurs in normal HDDs but that is not the sort of fragmentation we are referring to. The reason Intel SSDs are so much better in write performance is that they basically combine many sub block size writes into a full block write. The problem is when a sub block size file is deleted leaving intrablock fragmentation. Over time either the block needs to be rewritten entirely, which requires further write wear, or performance degrades. This is the problem which Intel have acknowledged and are currently seeking a solution for.