Rosh haShana 14b:
והתניא לעולם הלכה כדברי ב"ה והרוצה לעשות כדברי ב"ש עושה כדברי ב"ה עושה מקולי ב"ש ומקולי ב"ה רשע מחומרי ב"ש ומחומרי ב"ה עליו הכתוב אומר (קוהלת ב) והכסיל בחשך הולך
Thus, one who follows the stringencies of both Bet Hillel and Bet Shammai is the subject of the verse in Kohelet which states "but the fool walketh in darkness."
It is not enough for me to simply cite that pasuk without having a good explanation of why that pasuk applies to one who accepts these unnecessary double-stringencies. Why not apply the beginning of the verse, "the wise man, his eyes are in his head," and call him a wise man rather than a fool?
That is, how do we know that adopting these extra chumrot is considered walking in darkness, or that he should be called a fool?
Just as I explained for Rav Ashi's application of the verse in the previous post, I would put forth that this (like many other drashot) is based on word-play. Do not read the ש in והכסיל בחשך הולך as a shin but rather as the letter sin.
The word חשך, spelled with a sin, means "to spare, withhold, hold back." An example of this in Biblical Hebrew can be found in II Shmuel 18:16:
Rather, with the sin pronunciation, the meaning of the verse is:
"but the fool walks in stringencies," or "but the fool walks, holding himself back."
And thus we derive that (at least in this particular context) unnecessary stringency is not a positive thing but rather the mark of the fool.