Why should scientists balance open-mindedness with skepticism?

This question presupposes that scientists should cultive these traits. It is a philosophical, and not a scientific question. "Open-mindedness" and "skepticism" are not defined, and both are highly subjective terms.

What a scientist should do -- must do -- is be led wherever the verifiable hard evidence he is able to collect takes him. To do otherwise is to be doing something other than "science".