There's one possible reason, that it took me forever to think of, that came up several times. So many times, the murderer is a celebrity that he and Mrs. Columbo are both fans of. So in those instances, when he definitely suspects the person, he might keep her from meeting them because of the unpleasantness of that. In "Forgotten Lady," you hear that Mrs. Columbo couldn't make it to the movie screening, but you also find out (regardless of the actual ending) that he came to arrest Janet Leigh. So bringing Mrs. Columbo along would be out of the question. So, though it took me so long to think of it, it seems to me that there are at least a handful of scenes where she's left out for that reason.