I usually use healing spells or healing potions instead of help.
Well, spells and potions cost resources while help does not.
Sure it cost you an action, but unless you have a very powerful potion or spell or the enemy is very weak the party member will go down with the next hit anyway.
If you are cynical you could argue that attacking downed party members shows how clever enemies are because they know other party members can bring them up any time infinitely.
The AI has learned that focussed fire on downed party members increases your chances to win.
Or maybe enemies are programmed to attack the party member with least HP and zero is less than any positive value.
Prof. Dr. Dr. Mad S. Tist 
