Why do established television stars agree to appear in lame new shows that make them look totally inept?

The latest to participate in a trend that has sent, most recently, Dick Van Dyke and Mary Tyler Moore into immediate critical and ratings oblivion is Robert Guillaume in The Robert Guillaume Show (8:30 p.m., Ch. 4), premiering Wednesday on ABC for what is appropriately termed "a trial run." The trial is for the viewers.Guillaume, who became a star on "Soap" and sustained a well-deserved reputation for comedic ability in the later "Benson," would seem to deserve at least some of the blame in this case, as he is listed a co-executive producer with Phil Margo.

Guillaume plays Edward, a divorced marriage counselor - oh, the comic possibilities boggle the mind - with a smart-aleck father (Hank Rolike) who issues one-liners with all the charm of a gumball dispenser and is obsessed with his long-ago sighting of Big Foot.

Wendy Phillips plays Edward's secretary. Kelsey Scott and Marc Joseph play his teenage children so Guillaume can do his Bill Cosby father-of-the-year impersonation.

The pilot episode is taken up with Edward's search for a secretary - one applicant is a psychopath with a falcon on his arm, a bit that might do for "Monty Python's Flying Circus" but has no place in what aspires to be a family sitcom. The subplot involves his daughter's chagrin at being cast in a school production of "The King and I" as the king - enabling Guillaume to do his Yul Brynner impersonation.

Phillips is a likable actress who has been handed an unlikely character. In order to get a secretarial job, she hands in a phony resume that would seem to qualify her more for a professorship. The pivotal plot point is whether Edward will hire her anyway. She's a regular on the show. Take a wild guess.

-Elsewhere in television:

A KINDER, GENTLER, MORT? - What's this? A rather sedate discussion of John Tower's nomination to be secretary of defense - on "The Morton Downey Jr. Show"?

Sure, the conversation tended more toward speculation on the sexual proclivities and imbibing practices of members of Congress than the cost-effectiveness ratio of, say, the B-1 bomber. But at least there were no knock-downs or shouting matches.

Downey, making an apparent bid to be taken more seriously - and to reassure stations who might consider putting his mostly late-night syndicated talk show in an afternoon time period - sent a letter last month to station managers promising to tone down the language and concentrate on issues.

"We are dispensing with excessively harsh language across the board," said the letter. "I will not use it, nor will I tolerate it from any guest. Early in the development of the show, we saw ourselves primarily as late-night programming and would use strong language to spur reaction and discussion. Unfortunately, the critics have seized upon this issue to attack everything about the show. However, harsh language is not a necessary element of our show and it will be gone."

He also promised the show would get out of the studio to do remote segments and that he would involve more members of the audience instead of allowing a rather colorful group of hooting regulars to dominate.

In a telephone interview, Downey played down the changes, but did say he is concerned about becoming "a caricature of myself."

"You're going to see the same Morton Downey Jr. you've always seen - except I'll wear a tie and a jacket," he joked. But he said he was "in the process of retraining myself" to use less offensive language in his dialogues with guests, a couple of whom have been prompted to pursue legal action.

The show is still predominantly late-night, with most of the 84 stations that carry it running it in the 11-midnight timeslot. Downey says many stations are considering putting the show on in the daytime, however.