The Be'er Heitav, quoting the Bach, writes, "Those who are careful will always say the birchas hamazon from a siddur and not by heart."
Yalku Ma'am Loez (Bereishis vol.1 p.123) writes, "Say the tefillah from a siddur, because if he says it by heart, his mind will wander to other topics. But if he tries to say it from a siddur and doesn't raise his eyes, he will certainly not have foreign thoughts. If he will toil with all his strength to daven with good kavanah, Hakadosh Baruch Hu will certainly have compassion on him, and Hashem will save him from the yetzer hara. We heard about Reb Yitzchak Luria, the Arizal, that he would even say birchas hamazon from a siddur, although this is something that even children know by heart."
Reb Eliezer ben Makir (Seder HaYom, Birkas HaMazon) writes, "If he has a sefer before him to look into so he can have kavanah, this is better, and then he won't be sidetracked to think about other matters. נעים ומה טוב מה, this is certainly the best option. He will perform the mitzvah d'Oraysa according to halachah, and this will bring brachos to himself and his family."
A story happened in Yerushalayim, where a woman came to Reb Yehoshua Leib Diskin zt'l and expressed her tzaar that her children are niftar when they are still young, r'l. She asked Reb Yehoshua Leib to give her a brachah. Reb Yehoshua Leib advised her to be cautious to say birchas hamazon from a siddur. The woman listened to this advice, and her children survived, and she was able to build a family. (Written in Amud Eish, דיסקין ל"מהרי הגאון תודלות, p.161).
