Decals have an Opacity setting, but there isn't much of a real usage case for it, as Opacity, the way it works just fades the image.
Translucence however can be handy, without fading the image it would allow what's behind it to pass-through.
A specific usage cenario is applying images to transparent materials or making light boxes, where there's an emissive material behind and the light should pass throught the decal and lighten it up without fading.
Another option would be for the decal to effectively inherit the material characteristics it's applied on, it does for glossiness for instance, but not for transparency or emissivity,
