Inspired by the upcoming holiday of Pesach, where many minhagim seem to have each person drink from his own cup, I was wondering about the custom of drinking wine from the cup upon which kiddush is made on Shabbos (often not directly from the cup, but the same wine that has already been "blessed"). Is there any real reason to do this? We are blessing Hashem, not the wine, correct? (edit: is there possibly a problem vis-a-vis A"Z in treating the wine as blessed?) Why not have individual cups of wine pre-poured at every place, and the leader be motzei everyone? And if so, can the same go for challah?
The cup that was used for the mitzvah has a special blessing attached to it. In particular, the Gemara (Berachos 51b) states that with the cup of wine used for the Grace After Meals, it's especially important to give some to the lady of the house. (And it goes on to tell about how Yalta, the wife of the prominent Amora Rav Nachman, was extremely insulted when a guest who had recited Grace declined to do so.)
Aside from this, there is a halachic issue involved. While it is true that the listeners fulfill their obligation by just hearing kiddush, the best way to perform the mitzvah is for them to drink some of the kiddush wine (either directly from the cup, or by having the one who recited kiddush pour some for them). This is noted by Rashbam and Tosafos to Pesachim 100b (s.v. yedei kiddush), and codified in Shulchan Aruch, Orach Chaim 271:14.
While one listening to Kiddush doesn't need to drink ANY wine, one should preferably drink some wine - ideally Kos Shel Bracha. The Shulchan Aruch actually writes that one shouldn't drink from the same cup as another. According to the Shulchan Aruch Harav, though, if the wine was poured out into other cups before Kiddush, those are also considered Kos Shel Bracha.