The hierarchy T-Rex is talking about can be seen here.
Maslow's hierarchy of needs, and the other ideas put forth by the humanistic psychologists, were in some part a reaction against other leading psychological theories at the time, those of the behaviorists and the psychoanalysts. Those ideas seemed to reduce people as just fulfilling their base desires, or mechanically responding to stimuli. Maslow was trying to develop a theory of human personality that respected the broad, over-arching goals that many humans have throughout their lives: the desire to feel loved and accepted, the desire to fulfill our potential, the desire to experience the beauty in art and literature. These are often things we seek out on our own, for reasons the behaviorists and psychoanalysts cannot satisfactorily explain (except in those cases where we do these things just to get laid).
Like any theory, it has its flaws, though it doesn't seem to be nearly as flawed as psychoanalysis. As a therapeutic approach, it works just as well as any other therapy. It also doesn't have much predictive value, because plenty of people will be so driven by goals at the top of the pyramid that they will forget about goals lower down. One such example would be the starving artist who is so devoted and engrossed in their art that they stop bathing and stop seeing the people they know. Such a person is admittedly an "outlier," but there are plenty of other examples of people who ignore their basic needs and safety in order to fulfull a higher goal. People can be pretty, um, driven. I want to say "crazy" but that is not actually what is going on and would totally be misleading in this context.
And I think people do care about basic safety when they need to poop. They just might make compromises on it. Like, if you were in a war zone, and you had to poop, and your choices were 1) poop in your hiding hole, or 2) go out and poop in the line of fire, the second option is going to seem pretty attractive when you consider how disgusting the first option is going to be.
I want to stop their for the sake of style, but the philosopher in me wants to add in one last bit, which is this: there is a slight additional problem that the top of the pyramid (self actualization) does not have a whole lot of data to back it up. However, Maslow's description of what self-actualization is like seems to be remarkably similar to Heidegger's idea of "authenticity" (Eigentlichkeit) in his book Being and Time. People who are self-actualized have a developed sense of morality, a tendency to act spontaneously, care about problems greater than themselves, and are more accepting of themselves and others. Whether or not the general humanistic theory is sound, though, it at least provides some ideas of how we can be happy and satisfied in our lives. And I'm kind of okay with that.