IT'S one of those things that sounds like a good idea at first glance: Combine four government agencies into one, eliminate duplication, and save a lot of money. That's ostensibly what a bill championed by Sen. Jesse Helms (R), chairman of the Senate Foreign Relations Committee, would do. Senator Helms wants to merge the United States Information Agency (USIA), the Agency for International Development (AID), and the Arms Control and Development Agency (ACDA) into the State Department. The bill in question, which includes a whole grab bag of proposals near and dear to the isolationist wing of the GOP, is currently hung up on the Senate floor. Two attempts to close debate in early August failed to get the necessary 60 votes. A similar bill has already passed the House of Representatives. In its current form, the bill is unwise: It's basically a cover for Helms's longtime agendas of eliminating foreign aid and independent advocacy of arms control. Most of the savings would come, not from getting rid of duplicative administrative or functional costs, but by closing all AID missions overseas and slashing the agency's staff by 50 percent, along with cuts of 25 percent at USIA and 14 percent at State. The administration is tenaciously fighting the proposal and the president says he will veto it. But Helms has skewered the White House with the fact that late last year, Secretary of State Warren Christopher forwarded to the White House a very similar plan. After some ferocious lobbying by the agencies, Vice President Al Gore, in charge of ''reinventing government,'' turned it down. Here's what we see as the rationale for keeping the agencies at least somewhat independent from State: USIA: Through the Voice of America shortwave radio broadcasts in many languages and its other cultural activities, USIA is America's cultural and public-relations agency overseas. State didn't do a bang-up job when it was in charge of cultural exchanges from the 1950s to the 1970s, and the farther VOA can stay from the policy-formation folks at Foggy Bottom the more credibility it has. AID: While the agency has a reputation as one of the worst-managed in government, a lot of that is Congress's own fault. AID's modus operandi has been to turn over large sums of money to the field offices to spend as they see fit. While this ought to lead to maximum flexibility and efficiency, too often it has led to waste, fraud, and mismanagement as some third-world AID clients have found creative ways of spending development money to develop their bank accounts. Congress then responds with legislation that puts the agency in a management straightjacket. The policymakers at State can't fix that by themselves, and their record doesn't indicate they'll spend a lot of time worrying about the downtrodden at crunch time. ACDA: Often it does take a rocket scientist to do arms control. ACDA's experts have the required expertise; State's Foreign Service generalists, usually highly intelligent, do not. The Foreign Service career track intentionally punishes, rather than rewards, overspecialization. In fact, all three agencies provide a cadre of highly skilled specialists, which State cannot. They also represent independent advocacy that cannot be silenced within the State bureaucracy in favor of other issues, such as country-to-country relations. THAT'S not to say the status quo is hunky-dory. If the deficit is going to be trimmed, cuts do have to be made. But the administration, through the vice president's government reorganization effort, is making a serious effort to trim all four budgets. The Republican leadership should likewise focus more on intelligent budget trimming and less on ideologically motivated reorganization for reorganization's sake. A closer relationship to the secretary of state need not necessarily damage the agencies' independent voices. Right now each agency reports to the president and the secretary. One arrangement might be to make each agency head an undersecretary of state in charge of his or her respective agency. To continue to attract the experts the government needs, each agency should maintain its own personnel system. Other models might be a relationship between the agencies and State similar to that of the Joint Chiefs of Staff and the secretary of defense or that between the Coast Guard or the Secret Service and the treasury secretary. There is room for compromise on the issue if the administration and Helms really want it. In the meantime, the chairman should return to the mellow mode he adopted earlier this year and stop holding treaties and ambassadorships hostage to administration acquiescence. It's petty, and the nation has urgent business overseas that won't wait for domestic politics.