The Ksav Sofer offers a number of answers, but the one I liked most is built on a clever pshat of the Chasam Sofer as to why Mordechai was “ratzuy l’rov echav,” but was not looked upon so highly by the Sanhedrin after the events of the Megillah. What did they want from the guy – he saved Klal Yisrael!? So it might have entailed some bitul Torah, as Chazal tell us, but weighed against the danger of Haman, how can you fault Mordechai for closing his gemara to go do something about it?
Chazam Sofer answers that of course Mordechai was right to use his talent and position to save Klal Yisrael, even though he had to give up learning time to do so. But there are many paths to save Klal Yisrael that are open to Hashem. Doesn’t Mordechai himself tell Esther that if she doesn’t step forward, “Revach v’hatzalah ya’amod la’yehudim m’makom achier?” Yet the very fact that the hatzalah came through Mordechai, forcing him to take away learning time, and not through some other means, proved that somehow Mordechai’s torah, Mordechai’s learning, suffered from some shortcoming (obviously we are speaking about a fault on a level we cannot even imagine). Otherwise, says the Chasam Sofer, Hashem would have worked things out so that Klal Yisrael would be saved without disrupting Mordechai’s precious limud haTorah. This was the criticism of the Sanhedrin.
One might have thought that the person elected to be Kohein Gadol, who must devote time and effort to filling a leadership position, is someone whose torah and avodah in other areas is less valuable to Hashem, otherwise why would Hashem take that person out of the beis medrash? Moshe therefore tells Korach that in this instance, that’s not the case. The person selected by Hashem, “Hu hakadosh,” he is the holiest member of the group and is the most worthy.
Let me end off with a little question that I don’t know the answer to: so how do we know the difference? If a person is thrust into a position where he/she feels that he must take on a public role at the expense of private avodah and/or torah learning, how does a person know whether that means he is a Mordechai and at the end of the day will be only “ratzuy l’rov echav,” or whether he is an Aharon, “Hu hakadosh?” I don’t think a person can really know and perhaps it shouldn’t bother him anyway. Let Hashem take care of the cheshbonos.