In the case of Judaism, Israel makes Orthodox conversion more useful since it doesn't accept you as a real Jew if you are a convert to a more weak tea version of Judaism. People who convert are usually stronger believers, so they would convert to the more undiluted version of the religion anyway.

On the other hand, 'cultural' Jews are more likely to marry non-Jews, and absorb them into Judaism that way. And I suppose this leads to cultural Jews leaving Judaism too. I think I read somewhere that the descendants of all the founders of Reform Judaism ended up outside Judaism.