Hmm... I can see your point, but I'm not sure I want to go down this path. I allow similar behavior for demons because the situations in which you gain too many demons are usually a bit more chaotic and it is possible the player did not notice they were full on demons when they started the link (after all, this almost always happens in combat, presumably they had something else on their mind.
)
With abilities though, it is always the result of Copy Ability, Fuse Demons, or Revelation: there's nothing stopping you from using these in combat, but most players don't, so that reason doesn't really exist here. I think for now I'd rather stick with a demon actually needing to fully learn an ability before you can Copy it around.