The Shulhan Aruch (Orah Haim 191) writes that the Sages instituted an abridged version of Birkat Ha’mazon to be recited by workers who are being paid wages for a full day of work. Since their time belongs to the employer, the Rabbis allowed the workers to recite a shorter version of Birkat Ha’mazon. The workers recite the entire first Beracha, and then a brief combination of the second and third Berachot. The fourth Beracha, which to begin with does not constitute a Torah obligation, is omitted, and certainly the "Ha’rahaman" section is omitted, resulting in a significantly shorter text of Birkat Ha’mazon. This was done to minimize the amount of worktime taken away from the employer.
This Halacha does not apply nowadays, because, as the Poskim explain, employers today – thankfully – are not so strict that they refuse to allow their employees the several minutes needed to recite the complete Birkat Ha’mazon. The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) goes so far as to say that even if the employer explicitly stipulates that he hires the worker on condition that he does not recite the full text of Birkat Ha’mazon during work hours, this condition is not binding. Since the accepted practice today is to treat the full text of Birkat Ha’mazon as an outright obligation, the employer is not halachically permitted to make such a stipulation. Therefore, even if an employer makes this demand, the employee recites the full text of Birkat Ha’mazon. (Birkat Ha’mazon is similar in this regard to the evening Arbit prayer, which, strictly speaking, is optional, but has been accepted by the Jewish Nation as an obligatory prayer, and it must therefore be approached as an outright obligation. The same applies to the full text of the Birkat Ha’mazon, which has been accepted as obligatory and may thus not be substituted by the abridged version.)
Although this Halacha is not actually practiced nowadays, it nevertheless yields – albeit indirectly – a vitally important practical lesson. The Talmud Yerushalmi raises the question of why the Rabbis instituted a special abridged version of Birkat Ha’mazon, instead of simply allowing workers to recite Birkat Ha’mazon while working. Rather than have the workers skip portions of Birkat Ha’mazon, the Rabbis could have enacted that after reciting the first Beracha, workers should resume working and recite the rest of Birkat Ha’mazon as they perform their work. The Yerushalmi answers that it is improper to engage in any activities while reciting a Beracha, and so the Rabbis did not want to allow employees to work while reciting Birkat Ha’mazon.
The Mishna Berura (Rav Yisrael Meir Kagan, 1833-1909) finds it very significant that the Rabbis preferred having employees skip portions of Birkat Ha’mazon altogether rather than have them recite those portions while working. If these are the only two options, they felt it is better to arrange an abridged version of Birkat Ha’mazon. This shows us, the Mishna Berura writes, the extent to which the Rabbis deemed it improper to engage in any sort of activity while reciting a Beracha. This means, for example, that one should not begin reciting "Asher Yasar" after using the restroom while washing or drying his hands; he should begin the Beracha only after finishing drying his hands. Another common example is clearing the table while still reciting Birkat Ha’mazon. This is improper, as while reciting a Beracha one should not be doing anything else. This is crucial not only to ensure that one concentrates on what he is saying, but also as a sign of respect for the Beracha, showing that he considers it important. A doctor treating a patient should give the patient his full attention; engaging in other matters while tending to the patient would be very disrespectful to the patient and would show a gross disregard for his duties as a physician. Likewise, engaging in other activities while reciting a Beracha is disrespectful. Therefore, whenever we recite a Beracha, we must give the Beracha our full attention, and not be doing anything else.
Summary: In ancient times, workers were allowed to recite an abridged version of Birkat Ha’mazon, because employers did not allow them the time to recite the full text. Although this Halacha does not apply nowadays, nevertheless, the fact that the Rabbis preferred instituting an abridged text rather than allow employees to recite Birkat Ha’mazon while working shows us the importance of refraining from all activities while reciting a Beracha.