for a lot of people, ext4 is a pretty new filesystem, just now getting to the point where it has enough of a track record to trust data to.
I haven't benchmarked against ext4, but I have done benchmarks with the filesystems prior to it, and I've run into many cases where JFS and XFS are clear winners.
even against ext4, if you have a fileserver situation where you have lots of drives involved, XFS is still likely to be a win, ext4 just doesn't have enough developers/testers with large numbers of disks to work with (this isn't my opinion, it's a statement from Ted Tso in response to someone pointing out where EXT4 doesn't do as well as XFS with a high performance disk array)