EXT4 can support individual files up to 16 terabytes, and volumes up
to one exabyte in size. But, one of the aspects of EXT4 which
contributes to better performance, though, is that EXT4 can handle
larger extents-a range of contiguous physical blocks of data. This
allows it to work better with large files and reduce drive
fragmentation.

Other factors include the allocate-on-flush technique used by EXT4. By
delaying allocation of data blocks until the data is ready to be
written to disk, EXT4 improves performance and reduces fragmentation
compared to file systems that allocate blocks earlier.

Using checksums for drive journaling improves reliability and improves
performance by avoiding waiting on the disk during the journaling
process. When it comes to file checking, EXT4 is quicker because
unallocated blocks of data are marked as such and are simply skipped
during disk check operations.

Any discussion around 'why/why not' and 'defrag' needs to include Diskeeper (formerly Executive Software). Offering to license your product to Redmond, and having your company and/or product still be viable in two years, is a Herculean adventure at best ..