None of the daily/weekly/monthly tasks that run overnight do anything
resembling a defrag.
I didn't think so, but I have heard people (on the LUG I think) say that.
If you want your drive to be defragmented, you need
to do it manually.
I've never heard of Diskwarrior doing anything bad to a system.
I saw one guy who claimed to be an apple tech say that Diskwarrior sometimes causes problems - although it was a little unclear if he was referring to the newest X version of Diskwarrior (3.02). However, Diskwarrior doesn't defrag - but rather straighted out the file database.
Supposedly X defrags itself somehow. Perhaps its when you do a system upgrade and it says "optimizing system performance" which takes 30 minutes sometimes.