MB 3: Some permit - The responsa of the Ridbaz (Siman 532) goes through great lengths [to justify them] and permits them, see there. And see there where he cautions strongly against changing our custom. See also the Bach Siman 68 and the responsa of the Chovas Yair #238, who says that it is proper to recite Insertions and not to change our custom.
MB 4: Recite them - See the Pri Chodosh who warns that it is important not to delay past the proper time for Shema and the Amidah because of insertions, and also not to fast half the day by delaying the [Shabbos or Yom Tov] meal past noon because of them. One who is praying with a Minyan and sees that the recitation of insertions will cause a delay of Shema past the proper time should not wait for them; rather, he should recite the blessings preceding Shema and continue to say the Shema [in its proper time] until the end, and then wait for the Minyan to catch up. While he is waiting, he should remain quiet and not talk (even in Torah learning), until the Minyan catches up with him, and then should continue his prayers with the Minyan.