ESCHEWING THE nostalgic ground already covered by the two That's Entertainments. That's Dancing is not simply a collection of dance highlights. It's an ambitious attempt to touch on all the bases of modern dance--from break dancing to ballet. Accordingly, its five sections include clips from distant, lands and as distant times as film could record. But, of course, it's the precious leftovers from the That's Entertainments that make That's Dancing worthwhile.

Having forsaken the summit of the terpsichorean mount. That's Dancing, through various approaches, reveals the slopes to be full of unusual treasures. The brisk journey drags in only a few spots. Dancers, like athletes, often move better than they speak, and the introductions they offer to the five sections are generally useless.

The film begins with the voice of Gene Kelly, the chief narrator, over a dark screen, delivering a goofily pretentious lecture on the primal significance of dance. From there we go on a whirlwind tour of dance around the world and through the ages, bringing us to the present: Gene Kelly in a New York playground explaining the art of break-dancing. Unfortunately, Gene refuses to moon-walk or spin on his venerable head.

The ballet section, in particular, suffers from excessive talk, despite rare footage of the legendary Isadora Duncan and a brief clip of Rudolf Nuryev dancing with Margot Fonteyn. Presumably this is because ballet requires more explanation for today's audiences.

And some of the dancing is truly funny. The Busby Berkeley extravaganzas are a real joy, screaming with a "camera-as-new-toy" aestheticism. The footage from the "Stone Age," the early years of dance on film, features clumsy chorus lines whose tubby legs bespeak better cooking than choreography. Clowning aside, That's Dancing includes a superb dance by Eleanor Powell, the greatest female tapper ever who, because of her less-than-mediocre acting, never enjoyed the popular fame her footwork deserved. On the floor, she is simply astonishing. Another inclusion oft-neglected elsewhere is the Nicholas Brothers' rubber-legged vaudeville routine from Down Argentine Way. Shirley Temple shares a charming tap duet with the original "Mr. Bojangles," influential but rarely filmed Bill Robinson.

THE HEART OF the show, as everyone knows, is the glorious era of RKO and MGM musicals. Ray Bolger does the boasting for all the luminaries present: Gene Kelly, Fred Astaire, Ginger Rogers, Cyd Charisse, Eleanor Powell, Donald O'Connor, Judy Garland. Among the MGM treats is Gene Kelly's original dance with Carol Haney, whose figure was later replaced by a cartoon figure for the animated classic Invitation to Dance. Kelly dances with garbage-can lids on his feet in It's Always Fair Weather and Donald O'Connor helps him make a shambles of a linguist's office in "Moses" from Singing in the Rain.

And then there's Fred Astaire. When Asaire dazzles his way through "A Shine on Your Shoes." "Pick Yourself Up," and "Night and Day," . . . well, it's no secret: when God Himself dances. He imitates Fred Astaire.

One unusual highlight is a Ray Bolger dance which editors cut from The Wizara of Oz. Just before Dorothy and the Scarecrow hit the Yellow Brick Road, Bolger performs a comic dance in which he flies through the air and, with the help of reverse photography, catapults off rubber fences up and down the lane. Although, in retrospect, Gene Kelly wonders at its exclusion, this bizarre dance clearly clashes with the conventional dancing in the rest of the show.

In an apparent attempt to attract younger viewers, a lot of recent dancing has been included: break-dancing, Michael Jackson's "Beat It" video, material from Saturday Night Fever and Flashdance. A wicked suspicion lingers, however, that this material was included in the hope that, in this context, the old would be vindicated and he new would seem banal--for banal it certainly seems.

Undoubtedly the contempt-breeding familiarity of TV is partially responsible. But, beyond that, you do sense that something has been lost.

IT'S NOT JUST that John Travolta looks ridiculously dated when he gyrates in his polyester disco-duds, while Astaire and Kelly weather 40 years with undiminished grace. Or that Flashdance, for all its sensuous verve, is just another rock video.

The technical skill of today's dancers is undeniable. The controlled dexterity and energy of break-dancers is marvelous--but it is marvelous like Mary Lou Retton's gymnastics are marvelous. Dance is becoming a sport, impressive and sometimes exciting. Sadder still, it is sometimes as inarticulate as a professional athlete. It is no longer the rhetoric of the body, the nuanced expression of the person in a physical poem. Even the truly great dancing shown from West Side Story and Sweet Charity lacks the stamp of personality which characterizes the dancing of Kelly and Astaire.

Because they began their careers in comic theater. MGM's great stars developed a special knack for giving movement meaning, for transcending footwork. When Fred lures deeper and deeper into a dance with every step and eventually, casually, plops her in a couch, smirks, and backs away wiping his hands, their dance is as dramatic as any dialogue; their movement as eloquent as any speech. Something more than style and footwork fills the scene. No doubt, years hence, we will look back at the 80's and see dancers worthy to stand with Astaire, Kelly, and Judy Garland. With luck, they will not all be ballerinas and Russian immigrants.

On that score, That's Dancing will not advance the art of ballet in the United States, but that's not why MGM produced this film: they're making a buck off their past. And it's not a bad past. True, the very best moments of American dance are already in That's Entertainment, but the films between 1935 and 1955 are an almost endless gold mine; That's Dancing is still a rich yield.