You are aware you are questioning the cycling gods with this question. And cyclists are turning over in their graves, just imagining bikes with anything besides a proper lockring. You may even be seeing some of their ghosts.

Only problem I see is that you may have some rust on the inside of the steerer. Also perhaps some wobble from the stem the the cap would minimize, being I believe a touch smaller ID than the steerer. (I haven't checked that but it would make sense to have a nice clean smooth chromed surface to contact the jewel of a stem rather than a crude hacksaw cut steerer the may well not have been cleaned up much after cutting.)

All smaller issues than angering the cycling gods which could have serious consequences.

Ben