The presidential advisory commission that yesterday recommended deploying 100 MX missiles largely tossed aside the old reasons given by President Reagan and President Carter for why the missile is needed and how it should be based.

In the process the prestigious 11-member commission, whose goal was to produce national consensus on the hotly debated MX issue, may have opened the door unintentionally to an even wider-ranging debate about what the public should believe about the Soviet threat to this country and how to respond.

In the 1980 campaign and Reagan's first two years in office, two main arguments were used in behalf of the MX: that the existing 1,000 U.S. Minuteman land-based missiles had become vulnerable to a surprise attack by a growing Soviet missile force, and that the MX, which would be based in some unspecified new way to allow it to survive such an attack, was needed to retaliate.

But the Reagan-appointed commission now strongly acknowledges that the 10-warhead MX missile is needed mostly to give the United States at least in part the same kind of threat the Soviets already have to knock out land-based missiles and other well-protected targets.

And it says that it is all right to put the MX into the Minuteman silos because, while those silos are vulnerable to Soviet attack, the Soviets would face grave uncertainties in launching such an attack and could not do so without giving U.S. missile-equipped bombers enough time to get off the ground and retaliate.

Commission member William J. Perry said in an interview that, while the Minuteman ICBMs are vulnerable to attack, the consequences of that vulnerability have been overstated. Perry was chief of research at the Pentagon under Carter.

Similarly, at a news conference yesterday commission Chairman Brent Scowcroft, a retired Air Force general, said that "for the present, we believe that vulnerability is not so dominant a part of the overall problem to require other immediate steps" and that "we believe the Soviets must have serious uncertainties" about how they might fare if they launched an attack.

Scowcroft said it would be "difficult if not impossible for the Soviets to attack simultaneously" the U.S. land-based missile and bomber force because of the different flight times for Soviet missiles fired from land to hit U.S. ICBMs and those fired from submarines to reach bomber bases.

Commission member R. James Woolsey also said vulnerability is "a long-term problem, not a short-term problem."

Former secretary of defense James R. Schlesinger Jr., a special counselor to the commission, added that "it is important not to exaggerate the vulnerability question." He warned that there is a great difference between the accuracy of Soviet missiles measured in tests and during a war.

"The Soviets can never have a high degree of confidence that those accuracies would be achieved in a massive strike that has never before been tested," he said.

Specialists have long understood that the Soviets would have difficulty designing a successful attack on U.S. forces. But that sense of the matter was overwhelmed in the last several years by political rhetoric about who was to blame for the vulnerability of a single part of that force, the land-based missiles. The fleet of U.S. missile-firing submarines remains invulnerable.

Essentially, the commission has taken a broader view of the vulnerability issue. But there remain questions about whether installing the MX in the theoretically vulnerable Minuteman silos will put a hair-trigger on nuclear war in which either superpower may see an advantage in striking first in a crisis.

The commission, however, was unanimous that the MX is needed to start a modernization program to match the Soviet threat, to provide a continued deterrent to Soviet attack, to reassure allies and to induce the Soviets to move toward arms control agreements. It also sees the missile as necessary to remove even any perception that Moscow has a strategic edge.

The commission says that, while Soviet missiles theoretically can wipe out U.S. land-based missiles in a first strike, the U.S. Minuteman force "cannot inflict similar damage." To deter the Soviets, the United States must have the same capability to hit missile silos and underground command bunkers of Soviet leaders, the commission said.

The Soviets would question American will and have no incentive to negotiate arms agreements if the United States "unilaterally terminated the only new ICBM program that could lead to deployment in this decade," the report said.

While a large deployment "of several hundred MX missiles should be unnecessary," the report suggested that some "reconsideration of this" could occur "should the Soviets refuse to engage in stabilizing arms control and engage instead in major new deployments."

The commission stressed that the "overriding goal" of its recommendations was to move the United States toward development and deployment in the early 1990s of a much smaller, single-warhead missile which probably would be mobile to make it hard to hit.

The idea is to shift to missiles that are less attractive targets than a 10-warhead MX, and to encourage the Soviets also to move toward single-warhead missiles as a way of removing the temptation for either side to strike first. The MX is meant to convince Moscow that its big missiles are "a wasting asset."

But it is not clear that Moscow will be moved in that direction, especially since the Soviets already have over 600 relatively new and large multiple-warhead SS18 and SS19 missiles.

Thus, while the commission is emphasizing that the future lies with the small missile and is not seeking to focus attention on the MX, it will undoubtedly be the MX that gets the attention of Congress here and the marshals in the Soviet Union. The Soviets probably will believe that the most likely course for the United States without an arms control agreement is more MXs, once the first ones have established a precedent for being deployed in existing silos, rather than a new missile.

A main reason for recommending deployment now for MX, as Scowcroft said, is that "we don't have a small missile" and years from now, if it is developed, "it may have some of the same basing problem that have faced the MX over the past year."