It can really open doors to some neat effects, like real time detailed accurate AO type effects for those contact areas.
Is there a reason as to why it is only available on translucent materials?
There is a reason of course 
I’m not an expert of the rendering pipeline but basically translucent materials are rendered after the opaque ones which is why they have access to depth information while opaque materials do not.
For opaque materials there may not be any information available behind current pixel.
Ah i see, Thanks guys.
If they would fade, they wouldn’t be opaque anymore.
I meant to use not for opacity on opaque materials but for other things such as: Put it in the alpha of a Lerp node and tell it to colorize the intersecting areas accordingly, faking dynamic AO in a way, could be useful for eyes edges and other things as well that are moving and require additional dynamic accurate contact detail.
It’s not possible to do this simply because of how that node works, you could make an alternative using some renderer modifications which allow you to render opaque geometry in the separatetranslucency pass but I wouldn’t recommend it.
there’s also another way you could fake it, but you’d still have to use translucency in able to use the Depth fade.
You can then fake a masked material by ‘Ceiling’ all your opaque/slightly translucent values to 1, this way you only have 0 or 1 values going into your Opacity, similar to how masked opacity works.