A Little Theroy
This actually got covered in a university course on Human Factors Engineering (by Dr. Lewis Chuang, to give credit). The basic premise was this:
Each HF system can be represented using this triangle. It's form then shifts according to the restrictions, needs, etc. of the situation (much like a radar chart).
E.g. a system designed for the work place has a longer side on the performance part (productivity being more important), while safety and satisfaction only get as long as the restrictions like budget allow.
So in a safety-critical system, it would look something like that:
The restrictions of the environment inevitably take away the room for performance and especially satisfaction (imagine having multiple different security protocols you have to go through in a nuclear plant for example, which block productivity but ensures safety). The satisfaction of the user here simply can not be equally important as it would be for a product like a mobile phone, where it is the dominating factor.
Answer to your Question
If the visualization above is to be used to answer your question
How would you justify how the emotional state of a user is equally import given the context above?
One would have to argue that it is not possible to justify equality in this case.
But in my opinion there is an objection to make here. Simply because it is not the dominant factor, it does not mean that satisfaction of the user can be completely ignored. It does happen a lot in such cases, though, but that is simply lazy and bad design.
It is bad because it puts the so highly valued safety at risk by numbing the user down to such a level, that he is operating the system on a zombie level of mental activity. That means always pressing the same 7 buttons in the same order, just to activate some standard protocol. So when something happens that is not standard, the user is thrown off and is at high risk of performing a panic reaction, simply because he is not used to using his brain with this system due to prior mind-numbing.
So, a good design would push the satisfaction boundaries as far as acceptable, to make even a critical system at least slightly enjoyable to use and to ensure the presence of the user's mental awareness.