in over 30 years of using various flavours of Windows (and Linux too) I have NEVER needed to defrag a drive -- and on the very rare occasions I've experimented with this stuff it didn't seem to make an IOTA of difference.
I'm rubbish at deleting stuff so ran for a long time with a very full HDD, in Windows XP, which didn't auto-defrag. As the anti-virus software was updating its database, it would keep having to scavenge around for space on the disk and end up with a database file in about 10,000 fragments (which of course it kept having to refer to, nearly every time I did anything on the computer). In that circumstance, defragging did make a big and noticeable difference. Another example is defragmenting the pagefile and hibernate file (which you could do in XP using Sysinternals) when they also got fragmented due to lack of space, and again this made a big difference.
But I agree that it's probably quite rare that it makes much difference.