That is an interesting question, why is there a procedure between "Normal" and "Emergency?" If the plane had in fact been filling up with toxic fumes the only question we would now be asking is "why did it take so long to evacuate?" This is an example of normalcy bias in my opinion. The problem is that there are a lot of people and little volume in an aircraft so you don't have the same time to ponder whether you have a real situation as you do with a house or ship. Fortunately these events are quite rare so I would think that a company could easily absorb the costs of non-essential evacuations, and it does provide good practice -- and each event gives the engineers something to ponder when designing new aircraft.
You don't want your last thought to be "it is probably nothing..."