Q. Why is it that only some of the guest bother to bless their hosts at the end of a Shabbat or other meal, with the extended bracha mentioned in the Gemara, and they say only a short Horachaman, mentioned in sidurim by Birchat Hamazon?
A. Shulchan Aruch (O.H. 201: 1) rules that one of the guest should be asked to lead birchas hamazon and quotes the full long blessing mentioned in the Talmud (Brochos 46: 1). Mishna Berura (ibid. 5) cites Lechem Chamudos as wondering why indeed we have changed and only recite an abbreviated version in the horachamons. Siddur Yaavetz also questions the above and strongly recommends to recite the Talmudical extended blessing, since every single word in it, carries a very profound and significant meaning. His opinion is echoed by Ze Hashulchan (201: 1) and others. It seems to be the accepted minhag of many Sephardic communities.
However, Sholchan Hatahor (ibid.: 3) sponsors bedieved, the ones who rely on the short horachamon version. Aruch Hashulchan (ibid.: 3) maintains that it is different in our days, when everyone recites the blessing after the meals by his own, and don't comply by just listening to the selected guest leading the services.
Horav Shlomo Miller's Shlit'a opinion is that one may follow the established tradition of many generations to recite the short version only.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a
Important Numbers
