If you correctly partitioned the data files for exchange, there would never be a reason to do a disk defrag. Fragmentation happens because of the algorithm used to find space to write to disk. Fragmentation will never occur on a drive that contains a single file. Likewize, fragmentation will not occur when it's all sequential IO, and all the files are the same size. If you keep you logs only on a seperate spindle, all the logs are the same size and IO is sequential; the logs won't get fragmented. The database drive, if it only contains the edb and stm files, should have a very low fragmentation rate. If you seperate the stm and emb files onto seperate drives, they should not become fragmented.
The edb file, internally, stores data in 4K pages. Although pages are freed when items are deleted, the pages are marked free inside the database and the size of the edb does not shrink. Every night, during background cleanup, an online defrag of the database occurs; consolidating free 4K pages. An offline defrage of the edb actually removes the whitespace and shrinks the size of the edb. This is not necessary unless a large amount of mail has been deleted and is never coming back - ie you permanantly removed several hundred users from the server.
