In the last few weeks I've used the 38mm XL a few more times. I've found that the best usage of it is for a super-wide square image on 4x5 (4x4). I don't shoot 6x6 but if I did, this field of view is like if I shot a 22mm lens or so on a Hasselblad or Rolleiflex, or a 10mm lens on 35mm camera and cropped to 24x24.
Here's a few more images from various places using it:
At the end of the day I don't think it's really useful to shoot on 4x5, unless I'm specifically planning on cropping it to 4x4 or something. For the full 4x5 image the 47mm XL is a better choice. On 6x12 though it's a different story. It pairs nicely with the 58mm XL for wide angles. Also, the 38mm XL is supposedly a 120° lens, just like the 47mm XL, so it stands to reason it isn't actually wider after cropping, unless focusing closely.