Sometimes you have to run only one card because of the limited numbers of card in a deck.
This is mostly due to how much this card synergizes with the rest of the deck.
For example if you want to make a Shaman deck with mana tide totem i suggest you only take one, because you don't want to waste any more slots, better take a card that has more board influence.
Another example is Holy Fire, it is a good card to make damage and heal yourself, but it is high cost and has no board influence, its basically an expensive execute / heal. Many times you will have it in your hand but you won't use it because you have power words or Big game hunter and so on.
That means it is a strong, but very situational card which will prove itself useless in some situation and useful in others.
So in summary, I recommend to only use one when:
- The card is very situational
- It's effect is not always gonna work ( see mind control tech for example, very unlikely to be used twice in the same game )
- You already have similar card and don't want too many of that type ( see starfall for example, 2 swipes and a starfall is usually more than enough to clear board as druid )
I will eventually update my answer it anything else comes to my mind while playing