I think, in years past, scopes were rather unreliable and a see-through was important insurance. Firstly, they were more likely to fog back then and also, many were mounted on some form of drift, so they could get knocked out of line. Modern mounts and scopes are normally quite reliable, so the need is much less.
Sometimes you'll see one mount now days that has see-through provision, but the other mount, other end, does not. In part, I think this happens because objective lenses are getting bigger and there's no room left to have a see-through.
Another reason for a see through is that these are rifle-shotgun combinations, and you don't necessarily want to shoot the shotgun through the scope if you get a sudden chance. However, when I have done so with my Sodia BBF, it worked!
Older Drillings and rifle-shotgun over under's, (B.B.F.) usually had two see-throughs; more modern guns; only if you're lucky. Krieghoff Drillings tend to have a distinctive 3-claw mount arrangement and that rear mount centre-claw really does stop any notions of a see-through. When mounts do have them, usually the opening is quite small and so they're not especially easy to use if you're trying to line up the front sight in the vee-notch for a rifle shot. But as stated previously, if an animal is so close that it's just a big blur in the scope, then yes, they can be useful then.
