It may be fashion, but I wonder if there's more to it. What about hammer guns that don't don't have them - like this Grant from 1879:

If they were fashionable at one time, wouldn't you see one on an early gun like that (bar action, 31" damascus bbls, btw)?
And what about putting a gun with a through-lump back on the face?
Won't you have to file up the part of the lump coming through the action? And won't that leave a gap?
Joe - I wonder if the through lumps were fitted precisely enough to take some of the force off of the hinge pin? I don't know, but it seems like the fit on the two would have to be perfect in order for them to work in conjunction. And if the bbls are properly fitted on circle, is it really necessary (especially ona shotgun)?
Also, does a through-through lump allow you to use a hinge-pin with a larger diameter? I imagine it does. I wonder if this is why Rigby stuck with them on their rifles. Did they use them on their best-quality shotgun?
Lastly, removing the through-lump would probably save time for the actioner - one less hole to cut and file up, one less place to fit the bbls, etc.
Anyway - I think the issue is more complex. That's all.
OWD