THE SAME FEATURES that make immigration impossible as a legislative issue increase its intellectual allure. The arguments over its effects pit one fundamental view of economics against another.  They force the children and grandchildren of immigrants to consider whether the open policy which allowed their forebears entry has now been overtaken by events. They raise issues that a big, diverse democracy finds awkward to discuss, such as whether racial and religious “balance” should have any weight in public affairs. This tangle of concerns, rational and visceral, helps explain why America’s earlier immigration policy looks unenlightened in retrospect. It also explains why since 1982 Congress has been bogged down in its efforts to change laws that nearly everyone considers defective.