Rabbi Helbo was asked the following question by a community in the Galilee: Can the obligatory Torah reading be carried out by reading from ḥumashim in public? He did not know the answer, nor did several of the other Sages to whom he turned. When the question was raised in the beit midrash one suggestion was that this case should be similar to reading from a Sefer Torah that is missing one column, which is not acceptable. The Gemara rejects that comparison by arguing that in the case of the Sefer Torah, there is something wrong with the text in question. In our case, there is a full ḥumash with nothing missing. The Gemara concludes by quoting both Rabba and Rav Yosef as ruling that ḥumashim cannot be used for public Torah readings because it demonstrates a lack of kavod ha-tzibur – respect for the community.
Most of the rishonim explain the case of ḥumashim to be one in which the scroll was written properly with all of the requirements of a Sefer Torah; the only problem with it is that it does not contain the whole Torah, but rather contains only a single book of the Torah. While our Gemara is clear that such a ḥumash cannot be used, in a responsum the Rambam rules that in a case where no complete Sefer Torah is available, a Sefer Torah that is missing a word or letter can be used. His reasoning is that ḥumashim are certainly less desirable than a damaged or missing Sefer Torah, yet even use of ḥumashim is restricted only because of kavod ha-tzibur and not because there is something inherently problematic with them.
Most of the rishonim disagree with the Rambam and argue that the problem of kavod ha-tzibur that is raised with regard to ḥumashim is only true in situations where the ḥumashim are full and complete. Such ḥumashim are certainly better than a damaged Sefer Torah that is missing letters or words. The Rashba suggests that a Sefer Torah that is missing letters or words in one ḥumash cannot be used for public reading in that ḥumash, but can be used, if necessary, for the other ḥumashim.