I think that sometimes, the distraction is so distracting that the audience is focused on the distraction as much as what's happening on stage. If a child was screaming throughout Send in the Clowns, at a certain point, I think it would have probably been the central part of my focus rather than Bernadette Peters. I understand why she might want to soldier on through, but I think I would have wanted her to stop as well. I also think it must have affected her performance. It is a really emotional and poignant scene, and it was probably ruined.
In the Michael Sheen incident, an audience member says that 5 phones went off in the first 45 minutes of the play. I can see why Sheen would feel like he should say something with that many instances so early in the performance. Each incident was causing the audience to lose more and more focus.