Although I do think its a mistake to focus on the bighead aspect and its more productive to look at why people sometimes get themselves in a head space where they take no responsibility for their errors.
Ummm...I am focussing on the "bighead aspect" because it is the best theory I have for why people get themselves into that headspace. The "big head" is not always expressed overtly, but it seems to consistently be there in a lot of different forms of defensive behaviour.
Furthermore various comments that I have found suggest that knowing this tendancy is very useful in practice. Specifically being aware of the fact that perceived threats to egos generate often illogical defensive reactions helps in understanding and avoiding defensive behaviour in people. I have personally seen examples involving both myself and others where this seems to have worked. I have also had recent discussions with several people who I have known for a while are good at threading past potential conflicts, and been told that they know this theory already and use it conciously. And finally a number of comments that I have read through the years finally fell into place - upon subsequent re-reading I see how much good advice on dealing with people follows from this principle of human behaviour.
That makes me believe that the theory is more important to learn than any specific dynamic that it clarifies. Which is why I focussed on it.