The Talmud (Yevamot 6b) indicates that the Biblical requirement to revere the Beit Hamikdash applies to Har Habayit even in the post-destruction era. Practically, this entails treating Har Habayit with a degree of respect by not using it as a shortcut, not entering it while wearing shoes, not spitting there, and other similar acts. This is derived from the fact that the Torah (Vayikra 19:30) juxtaposes Shabbat observance and temple veneration, implying that just as keeping Shabbat is timeless, showing reverence for the Beit Hamikdash is eternal. Rabbi Aryeh Leib Ginzberg (Turei Even Megillah 28a) reasons that extending this commandment to the post-destruction era necessarily means that there is still holiness (on a Biblical level) to Har Habayit (an issue that is the subject of debate in many tractates – see Megillah 10a and Zevachim 107b). In other words, Rabbi Ginzberg assumes that if there is a requirement to revere Har Habayit, it is due to its retained sanctity.
However, if reverence for Har Habayit obviously implies sanctity of the space, it should suffice for Rambam to rule that there is an obligation to fear Har Habayit even after the destruction, and then we could logically deduce that Har Habayit’s sanctity persists. However, Rambam (Beit Hab’chira 6:14-16) rules that the holiness endures forever based on a separate biblical verse and still relies on the verse cited in Yevamot (6b) in declaring that we must fear Har Habayit even post-destruction (7:7). The Birkat Avraham on Yevamot 30 asks why did Rambam find it necessary to quote two verses – one to teach the sacredness of the Har Habayit and another to teach the obligation for reverence – if one verse would have been adequate?
When it comes to the various laws and practices related to the Beit Hamikdash, there are two categories we can identify. Some laws flow from the sanctity of the Beit Hamikdash, while other laws pertain to the edifice itself. For example, Rambam (Beit Hab’chira 6:15) writes that the ability to offer and consume sacrifices depends only on the holiness of Har Habayit and is therefore possible even in the absence of the physical edifice (at least in theory). The issue of offering sacrifices in the post-destruction era has been the subject of much debate, especially in the last two centuries. However, Rav Soloveitchik pointed out that the commandment to appoint guards for the Beit Hamikdash, for example, depends not on the sacredness of the Beit Hamikdash, but on its physical existence. Rambam does not list this as one of the commandments that can apply today because the sanctity of the Beit Hamikdash persists. Furthermore, he writes (8:1) that guarding the Beit Hamikdash is a positive commandment not because we fear enemy attack but because it is a sign of respect – “a palace without guards pales in comparison to a palace with guards.” Rambam implies that the sentries add a degree of pomp and circumstance, making a visit to the Beit Hamikdash an experience. This impressive experiential component depends on the Beit Hamikdash standing in full functional glory, not on the holiness with which its location is endowed.
With this distinction in mind, we can return to our original inquiry and explain that the reason an additional verse is necessary to teach that reverence applies even post-destruction is that one might have thought that this law depended on experiencing the Beit Hamikdash in its full grandeur, and would thus be limited to a time when the Beit Hamikdash stood. The second verse teaches us that this commandment is not a function of the Beit Hamikdash standing in all its magnificence, but rather a function of the holiness embedded in Har Habayit.