Just from my experience (and I play poker for about 6 years by now) and therefore imo emotions have a bad influence on your poker game. Since I'm an impulsive person myself I really had a hard time for years. Even if I consider myself highly poker educated I would say that emotions are the reason why I'm not as successfull as I wish to be.
Example:
happy about being that good (in terms of poker decission) lead to a more loose play (losing) because you always think you can play the flop better than others. Poker is a game of small edges and those marginal and merky spots are really really tough.
Having several downswings. -> lead to change of my game ( a mistake by the way) because you concerned about that you play wrong.
Taking revange on another player is really really bad too.
I probably could name you thousands of examples out of my experience, as a recommendation why emotion should be eliminated from one's game. So I fully agree with the authors of the book and imo you should strive for of beeing emotionless during a game/session/...
It took me years to achieve this goal in 95% of the time and it is actually running better since.
@Saruniks: I don't think optimal stress is needed. Since money should not actually be an issue at all. If you start thinking about money you invested either way you aren't making optimal poker decisions and that's what a winning player should aim for -> making as many right decision as possible.