The truth is 150,000 years of recorded anthropological evidence (discarding recent discoveries) reinstate that the first grouping of man was "Hierarchical" hunting teams. These resembled Wolf-packs. Most mammals have an Alpha Male. Lions, Wolves, Elephants and a whole list comprising both predator and prey all have an alpha-male and hierarchy. Looking closer to our Primate family, there is hierarchy.
If we work further down the order, we find that our Military is the best response force and the last resort for every state. The moment social order breaks (which may be a blend of hierarchy and peer-to-peer networks), imposition of Hierarchy has shown to restore order. This entire argument might seem irrelevant to the topic, so I'll get back.
Flat teams are a farce. Every Organization has a creator, a leader who creates a sub-group who share his/her vision. The hierarchy might work at multiple levels, but they are usually pyramidal in many ways. "C. K. Prahlad" writes in his book "Fortune at the Bottom of the Pyramid" that Fast Moving Consumer Goods (a.k.a. FMCGs) are the way the lower ranks of society tap into the mainstream economy. Society in itself is hierarchical. I've worked in an outfit that claimed to have a flat team, but had an implicit hierarchy.
My message is no matter what your employer might tell you, there is Hierarchy. If they mention a "Flat Team" (not Open Communication), they imply hierarchy that is ill defined. It is better to work in a company with a well defined Hierarchy, rather than one that is confused in expressing the same.