In a recent issue of Jewish Action, R. Reuven Spolter wrote about the "brain drain" in the Modern Orthodox world in which many of the most talented and promising leaders are moving to Israel, leaving America with a dearth of leaders for the next generation. The current issue (Fall 2004) has a number of letters on the subject.
One writer writes that "[t]o be so concerned that the future of Modern Orthodoxy in America is in jeopardy because an incredibly small number of idealistic, young rabbis and congregants are making aliyah is to deny everything that we have yearned and prayed for over the last 2,000 years." Another writes that R. Spolter is "advocating a new brand of Modern Orthodoxy where adherents are serious about halachah... but not so serious about their commitment to Israel and to mitzvat yishuv Ha'aretz." And the letters go on.
What these writers entirely fail to recognize is that there are millions of Jews in America with little or no connection to Torah. If every observant Jew were to move to Israel today, these millions of Jews would be lost forever. If 99% of frum Jews moved to Israel, with 1% remaining behind to influence the non-observant, and that 1% is the correct 1% with the requisite skills and talents for the job, the benefit to Torah and Yiddishkeit would be immeasurable. Instead, what is happening is the exact opposite. Many of the people with the skills to revitalize American Jewry (including some of those letter writers) are moving to Israel and the rest of us (myself included), who lack those skills, are not moving. The results are disastrous, as R. Spolter showed in his article.
R. Hershel Schachter likes to tell the story of how one time R. Avraham Shapira, at the time Chief Ashkenazic Rabbi of Israel, came to speak at YU and had a private conversation with R. Schachter in which he tried to convince R. Schachter to move to Israel. After R. Schachter finished listing his many responsibilities, R. Shapira told him that he was obligated to stay in America rather than abandon his flock. R. Schachter wrote in his article on Yishuv Eretz Yisrael:
For individual Jews there may be extenuating circumstances... in which it becomes clear that their place in not in Israel. Based on this rationale, it has been suggested  that the greatest scholars and leaders of the diaspora may be permitted, or even oblgiated, to remain in Chutz LaAretz. Since their sphere of influence is in Galut, and it is there that they will ahve the most beneficial effect in disseminating Torah, we cannot say that there place must be in Israel.R. Yisrael Kagan, the Hafetz Hayim, told one of his sons-in-law that the time had come for the latter to take administrative reponsibility for the yeshiva in Radin. The latter objected, saying that he could not give up his time for learning Torah. The Hafetz Hayim responded that we are obligated to love G-d with all our hearts, all our souls, and all "me'odekha." The Sages tell us that me'odekha refers to our money, that we are obligated to sacrifice our money for G-d's commandments. The Hafetz Hayim said that me'odekha really refers to whatever is most important to us. To many people, that is money. But to others, it is their time for learning. We are obligated to sacrifice that which is most important to us - in this case, one's time for learning Torah - in order to do what is needed of us. The Hafetz Hayim's son-in-law gave up the most important thing to him for the sake of G-d. Others, to whom, perhaps, living in the land of Israel is the most important thing, might be challenged to give that up for the sake of the greater good.
 This was told to me by the Ashkenazic Chief Rabbi Avrohom Shapiro