About 3 decades ago R Hecht and I had a conversation over bout the M"B's tendency to mix in Mussar into his Halachic commentary...
In a recent M"B Yomit (8:26) the M"B stresses the Mussar of wearing Tzitzit out. He cites no sources in Sha'ar Hatziyyun, though he does cite several ma'amarei Hazal that might lend support to his thesis. WADR, I would choose to see these supporting the importance of treating Tzitzit with respect and even mindfulness, and not necessarily requiring them to be worn outside at least when wearing a Tallit Koton.
BE"H, I will try to find other specific examples of the same while going through the Sefer.
Best Wishes for 5773!