I think there is some truth to that (although I'm speaking about people proposing new ideas rather than making legitimate complaints). For me, the intent is respect - a way of acknowledging their input without direct rejection. The "I'll look into it" statement is often a more polite way of saying "I'll take the next steps if I think it is necessary." Often you already know enough to immediately say "no," but reframing it as "I'll look into it," helps show respect to the person asking by not immediately putting them down (and often in a public setting).

I realize that the value of this approach is highly dependent on a culture of public respect. Common in Japan, and common in some cultures in the U.S. Making the wrong person look stupid is a recipe for career problems in those cultures.