Sometimes mirrored UVs is desired. But the Relax Tool inside the Unwrap UVW modifier does not work correctly with mirrored UVs - the tool tries to "unmirror" it, which essentially messes up the UV layout.
Many times it's desired to have UVs in place as much as possible, such when aligned to a texture. But flipping the UVs to be able to relax them throws them around where they should not be, and any visual confirmation whether the relax operation improved the UV layout or not becomes very difficult to manage.
It would be very useful not to have to go through the tedious process or unmirroring UVs when relaxing. It just makes things much more difficult than they should be.
