It may be a credit to the relative efficiency of the nation's income tax system that someone almost always is suggesting it be used for some sort of side job, beyond the mere collection of tax revenues.

Whenever any other government program breaks down, the first instinct often is to suggest using the tax system to take its place. For example, if federal scholarship aid isn't keeping pace, why not enact a tuition tax credit? It sounds simple, at first blush.

A case in point is the longstanding proposal to replace the present muddled welfare and food stamp system with a so-called "negative" income tax. The idea has been around since the 1950s, and once again is being studied seriously.

The way the plan would work is that welfare benefits would be distributed automatically through tax system.. Every American, rich or poor, would file a taxx return. Those earning above a certain level would pay taxes. The rest would qualify for regular cash payments instead.

The idea orginally was proposed by liberals, and then embraced by conservatives in the late 1960s. While it's been endorsed by both groups, the plan never really has gotten off the ground - primarily because most middle-of-the-load lawmakers still are suspicious of it.

Supporters argue that the plan would prove simpler and more efficient than the present welfare system, with less red tape and bureaucracy. It also would provide some benefits to low-income workers - eliminating the abrupt cutoff that now discourages recipients from getting jobs.

Congress took a first step toward such a system a few years ago, enacting a new "earned income" credit that provides tax credits or direct cash payments to the working poor. But this provision applies only to families with children. It doesn't help singles or childless couples.

The negative income tax idea has surfaced again as part of President Carter's new welfare "refrom" plan. Carter has proposed using a variant of the negative income tax to replace some components of the present welfare system. A good many experts are backing the move.

Now, a series of new studies on the first actual experiments with negative income tax plans in various parts of the nation has begun to appear and is giving analysts pause over how well the tax proposal itself would be likely to succeed.

The studies, which deal with experiments conducted in Seattle, Denver, New Jersy and Pennsylvania, show that the negative income tax idea has possibilities. But they also have uncovered some unexpected problems. Among the results:

While the negative income tax eliminated the red tape of the present welfare system, it also prompted recipients to cut back the number of hours they worked at regular jobs each week. Men reduced their work weeks by 6 percent, women by 12 to 17 percent.

The greater financial indpendence the program provided increased the breakup of marriages among poor families. (Since the tax plan provides cash benefits to all persons, married or not, there was no need for couples to remain together when they didn't want to.)

The increased security reduced the likelihood that the recipient would stay on a job. Researchers reported that persons taking part in the negative income tax experiments left their jobs sooner than others and took longer to accept new jobs. So, the labor supply declined.

Despite the added efficiency, the cost savings were marginal at best. To scale benefits high enough to get poor families up to the $6,000-a-year "poverty" line would cost an estimated $20 billion to $30 billion more than the present welfare system.

Analysts point out there also are substantial problems involving enforcement. While the present welfare and income tax systems tolerate a significant amount of heating, the profusion of rules and regulations serves to make it more difflcult.

Experts say a full-fledged negative income tax program would open the way for even more fraud by eliminating these bureaucratic barriers. The question is whether the increased simplicity is worth the cost of stepping up enforcement efforts.

While these caveats, admittedly, the serious ones, tax planners say are serious ones, tax planners say they don't necessarily spell the end of negative income tax idea. The plan still is an appealing one, at least in terms of simplicity.

The problem is, how can you patch up the holes?

Richard Nation, a Brookings Institution welfare expert who worked on the Nixon administration's ill-fated welfare "reform" plan, suggests that part of the difficulty can be eased by linking the negative income tax with a guaranteed jobs program to make poor persons less dependent on payments.

Nathan is supporting a Senate measure that would offer a federally subsidized job to at least one of the parents in a family on welfare in cases where a recipient has gone job-hunting for 90 days or more and been unable to find work. He estimates that 175,000 jobs would be needed.

Others have suggested cutting back the cost of a negative income tax program by holding benefits to below the full "poverty" line or using the tax plan to replace only part of the present system. Almost every suggestion in the hopper involves tradeoffs of a sort.

What it all means is that despite the initial attractiveness of the proposal, using the tax system to replace the present welfare mess isn't the breeze some proponents thought it would be. That doesn't kill the negative income tax idea. But it may slow it down.