The Talmud describes a berakhah to be included in birkat hamazon for a guest to bless his host (See Rokeach, Dinei Berakhot 319, and Gilyonei HaShas).. It is for this reason that the guest is generally honored with leading birkat hamazon, so he should have the opportunity to recite this berakhah. Rav Kook (Resp. Orach Mishpat, 42) notes that as contemporary practice is that every individual recites the birkat hamazon personally, there is less of a need for the guest to lead. The Mishnah Berurah (201:5, Sha’ar HaTziyun 14) implies that the “Harachaman” in honor of the hosts fulfills this purpose. R. Moshe Sternbuch (Resp. Teshuvot V’Hanhagot, II, 122), who notes that therefore even one who stops at “yechasreinu” on Shabbat must recite this harachaman. R. Sholom Krauss (Resp. Divrei Sholom, IV, 16) observes that one need not be concerned that this is considered an inappropriate supplication on Shabbat. As a rule, those are discouraged on Shabbat because they direct one to worry about their personal needs and evoke worry; in this case, the prayer is directed toward another, and one who clearly has enough already that he is able to host guests (and thus not a case that will evoke worry).