Why Reagan takes a shine to the mini press conference

President Reagan has held his second impromptu press conference within a matter of a few days.

He evidently intends to keep these spur-of-the-moment, brief question-and-answer sessions with the media going on a fairly frequent basis.

The short notice involved in calling the two conferences meant that many White House ''regulars'' missed them both. There were some grumbles and protests from those left out.

But Mr. Reagan and his senior aides have, for some time, been seeking a way to work out a meet-with-the-media format that will allow him to respond quickly to pressing questions and yet not subject him to the range of questions that come during his monthly, half-hour press conferences.

By holding these sessions frequently, White House aides say they expect to prevent a big backlog of questions from building. Currently, the President must spend hours in preparation for the monthly press conferences. The frequent sessions he now is setting up will relieve him of that kind of preparation.

Thus, as he did on Wednesday, April 14, in the Rose Garden, Reagan is expected to leave time only for the questions that are foremost on the thoughts of his questioners -- like the Falkland Island situation and the budget. On these subjects, Reagan aides point out, the President is ''up to speed'' because he is dealing with them on a moment-to-moment basis.

On top of this, some people in the administration say Reagan deals better with the press when he responds to reporters briefly and frequently -- when the hot lights of live television are not turning the exchange into a widely viewed program where Reagan must so fiercely watch his words and the way he expresses himself. Reagan is considered by some of those around him to be more effective during informal sessions with the press than during formal press conferences.

Actually, the latest of these 10-minute-or-so sessions moved somewhat beyond prime national and international topics.

After Deputy Press Secretary Larry Speakes said, ''This has to be the last question,'' a reporter spoke up: ''Ed Rollins (White House political director) says that Republicans on Capitol Hill who don't support your program should be disciplined. Do you agree?''

The President: ''No, I don't agree. And I intend to support as many Republican candidates as I can in the coming election year. . . . I have never used anything or attempted anything but to try and persuade them to my viewpoint , and there's never been any club held over any of them.''

Later, Mr. Rollins, when hearing of the President's response, retreated not one iota. He insisted that GOP congressmen who are undermining the President should not be helped by this administration.

Questions were raised at a subsequent White House press briefing as to how the President would deal with Rollins. The answers were vague. It appeared that the President would maintain his above-the-battle stance while still permitting his political operative to do what he feels he must do in the field of practical politics.