Covering the mobile land ICBM and sea-based cruise missile under an agreement outside the main arms control treaty will prove of "major value." Paul C. Warnke predicted yesterday in his farewell press conference as director of the Arms Control and Disarmament Agency.

Warnke, looking back on his 20 months of heading the government's effort to negotiatate a second strategic arms limitation treaty with the Soviets, said that "if you had to make a decision" on those weapons today, the United States probably would go ahead with them.

But, by postponing their deployment under a three-year protocol appended to the SALT II tready, they will be taken out of "the heat of negotiations" so their implications can be fully assessed, Warnke contended.

The decision on the new mobile missile called the MX should not be made, he said, becuase U.S. planners still have not figured out a sensible way to deploy that nuclear blockbuster.

Proposals advanced for the MX missile include playing a shell game by putting each one in a field of several holes, the so-called MAP, or multi-aim point system; hauling them back and forth inside an underground tunnel; deploying them at sea, or carrying them aloft in airplanes.

Critics, including Warnke, have said the MAP system would make it difficult to verify how many missiles were deployed, the trench poses both protection and environmental problems; going to sea would not upgrade the land portion of the U.S. strategic missile force, and MX planes could be destroyed before they look off.

"I don't put a great deal of stock" in the theory that the United States' present force of 1,000 land-based Minuteman missiles are dangerously vulnerable today or are likely to become so in the 1980s.

Any Soviet planner who conlcuded that he could wipe out the Minuteman force in a surprise strike "would have lost his head," Warnke said. Moscow would have to assume that each of the untried Soviet warheads would work with "exquisite" precision and that the American president would "sit calmly by" rather than launch the Minuteman force before it could be destroyed by the oncoming Soviet missiles.

The United States should "continue to think about the concepts" for deploying land-based missiles like the MX missiles "but not go ahead until we find one that makes sense," Warnke advised.

President Carter is experted to take that approach in the fiscal 1980 defense budget now in preparation by recommending full speed ahead with the MX missile itself but postponement of the decision on how to deploy the weapon. The Joint Chiefs of Staff favor MAP.

Turning philosopnical as he talked with reporters in his State Department office, Warnke said that giving up weapons in a "very unnatural act and that is one reason it takes a long time" to negotiate an arms control agreement.

Each weapon put up for discard from the nuclear arsenal, he said suddenly" looks better than ever before" and thus "becomes harder and harder to give up."

However, "We're now so close" to completing the SALT II agreement "that to me it's inconceivable that the two sides can't complete it sometime in the near future."

Not only is Warnke leaving his job as director of ACDA and chief arms control negotiator, a job he officially leaves today, but his Soviet counterpart is departing as well.

Warnke said that Vladimir S. Semyonov is leaving as head of the Soviet SALT delegation after nine years to take a diplomatic post in Bonn. Semyonov will be succeeded by Victor Karpov, a member of the SALT delegation.

"I respect him very much," Warnke said of Semyenov, calling him, "avery proper man" who sometimes scolded Warnke for talking with "too much vehemence" at the SALT negotiations.

Warnke was sworn in over considerable Senate opposition on March 14, 1977, and will return to private law practice in Washington with plans to serve as a consultant on SALT matters.He called the SALT II agreement a big step toward effective arms control," adding: "I like to think the treaty is a little better because I've been a participant."