It is one thing to regurgitate a widely believed aphorism, but an entirely another thing to justify it with facts.
Primordial instinctive fear is not required for the human species. Name one example where it is required (where an intelligent response is deemed insufficient).
1. Here is an example of reckless behavior associated with a lack of fear as it pertains to humans taking anti-fear drugs.
[1] http://news.nationalpost.com/news/there-was-no-fear-anymore-...
And then there are studies of animals changing their behavior to be more reckless. I am curious to see your thoughts on correlation of animal and human testing.
[2] https://med.stanford.edu/news/all-news/2011/03/scientists-di...
If one is going about life "without thinking or caring about the consequences of an action", that is more a sign of being unintelligent (quite often a case of native intelligence being hampered by such strong emotions as ideological feelings) than merely being fearless.
Animals (especially early primate ancestors of humans) do not have fully developed intelligence, so it not unsurprising that they unlike us require primitive emotions such as fear to survive as a species.
When one considers ill effects of fear, often stuff like PTSD and phobias come to mind. But normal people too are adversely affected by fear on a daily basis. As this documentary shows the masses are easily influenced (via fear) by propaganda/ public relations to make decisions that are not in their own best interest.