With the current Demonic effect, there is indeed no ill-effect for playing a 0-cost card with the keyword.
The reason cards would have the Demonic keyword even though playing that card wouldn't trigger any effect is for the benefit of other cards that count keywords in hand, check for neighboring or matching keywords, check previously-played card's keywords, or target keywords for discard. Look at Horde as another example of this. A 0-cost Horde card will never cost more recruits than the matching card, but it is useful in-hand for neighboring Horde cards.