Breishis 18:8 says that Avraham served the angels milk and meat together. Rashi in the name of the Medrash says that it only seemed like they were eating. Medrash Shocher Tov Tehilim 8 says that one of the arguments Moshe Rabbeinu had why the Torah should be given to the Jews and not stay in Shamayim is that the angels ate milk and meat together. According to this Rashi they did not eat, so what was the argument?
The simple answer is that that wasn't one of the arguments of Mosheh Rabeinu: It isn't mentioned in the gemara (Shabbas 88b-89a).
Addition: I saw that you added the source in the Midrash Shocher Tov which I previously hadn't seen. According to this, I think there is still room for two answers:
Rashi chooses contradictory midrashim (the approach often taken by the Mizrachi, e.g. in his commentary to the beginning of Bereishis)
Maybe the problem was מראית העין (though on the other hand, they didn't look like angels anyway).
I just learnt a Maamar of the Lubavitcher Rebbe as printed in the weekly Dvar Malchus booklet (see p5) and there, the Rebbe points out that it is mashma from the Zohar [Parshas Toldos 144a] that the angels actually ate , because:
1) The Midrash which Rashi cites [Bereishis Rabba 48:14] learns a lesson from the angels that when you come to a city - you should follow the customs of that city, and then ends:
but were they really eating? no, they only looked as if they were eating.
however the Zohar - after learning the same lesson omits the end (but were they..etc.)
2) The Zohar compares the angels eating to Yaakov's wrestling with the angel, where there too the wrestling was physical even though angels are spiritual.
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@Gershon, I just looked at the link you provided, and there it states
כשעלה משה לקבל פעם שניה את הלוהות ־ אמרו מלאכי השרת רבש׳׳ע והלא אתמול עברו עליה שכתבת ב־ (שמוה כי) לא יהיה לך אלהים אהרים (כ) אמר להם הקב״ה בכל שעה קטיגטין ביני ובין ישראל והלא אתם כשירדהם אצל אברהם אכלתם בשר בחלב שנאמר(בראשית י״ ח ) ויקה המאה והלב ובן הבקר .
so firstly: it was actually Hashem that made the argument for the Jewish people as to why the Torah should be given to them.
I don't think you have to say that the 2 Midrashim are contradicting one another.
Rashi mentioned that the 'men' were actually angels. So in order to answer the question that a 'ben chamesh leMikra' would ask himself: "since when do angels eat??" ... Rashi brings the Midrash that it only looked like they were eating.(So to a regular person - it looked like they ate...but what actually happened to the food.... that's another story ;))
But as far as Hashem is concerned [by Hashem - Emes - there's no 'kunsim'] (as per Midrash Tehilim) even though the food may not have been digested like regular human beings..... they are considered to have eaten it.