If your opponent is half-smart enough to realize you're probably going to switch out your Ninetales when standing in the rain, your opponent can switch back to Scizor as well. You are now losing the weather-wars in that all your momentum has left you. You can switch into another mon and then back into Ninetales, but likewise they can switch into Scizor and then Politoed, so you're at a loss for keeping the sun up. Now let's assume we don't even have Rapid Spin. Our momentum is not only lost, but Stealth Rocks are chewing out Ninetales on each switch in, meaning Politoed will eventually win out. We could switch to something else with Rapid Spin, but that still means your opponent still has a couple of turns to set up whatever they please while Ninetales and CAP5 are sitting out in fear. It becomes an ugly situation. All because we can't make the opponent wary of the potential of a Fire-type move. Give CAP5 a Fire-type move, and no longer can the opponent just send in Scizor and U-turn without a care in the world. We lower Scizor's usage, we heighten sun's usage, and we start to balance the weather war situation. Profit.