Not long into "Let's Spend the Night Together," the Rolling Stones rockumentary that opens at six area theaters today, one starts asking: "Why?"

Actually, there are several whys.

Why one more film of a gargantuan Stones tour? We've already got the compelling "Gimme Shelter," the 1970 Maysles Brothers film of the tour that put the cap on the Woodstock era with the disaster at Altamont. We've got "Ladies and Gentlemen: The Rolling Stones," the 1974 film of the 1972 tour. And now we have a 1983 film of the 1981 tour. Perhaps the director is Jumping Jack Cash?

In fact, the director is Hal Ashby, and his name is almost as prominent in the ads as that of the Rolling Stones. Again, why? The cinematographers and editors probably deserve more credit that Ashby, because nowhere is a director's hand visible. Ashby is no Martin Scorcese turning the Band's "Last Waltz" into a highly scripted, intensely personal recollection of a very special event. In several of Ashby's films--notably "Harold and Maude," "Coming Home" and the Woody Guthrie biography "Bound for Glory"--music has been a motif, an undercurrent or a motivation.

Here it's the only and the everything, and it's simply not enough. It's as if Ashby has inherited the passive Peter Sellers role from his own "Being There." He chronicles without involving himself--or the audience. He assumes it's enough to roll the cameras and point them at what he obviously thinks is the Greatest Show on Earth. If only it were.

One of the disturbing aspects about this film is that it's going to do little to support arguments for the Stones' exalted reputation. The only person who gives 100 percent is lead singer Mick Jagger, who always was the excitement factor, anyway. Twenty years on, Jagger is eerily unchanged, slight yet tough, still thriving on elbow-pumping, rooster-strutting energy and the ambiguous but heavy sexuality encapsulated in the world's most famous lips. He carries the load in "Let's Spend the Night Together" because he's the only one who seems to enjoy being there.

If Jagger remains as much the fashion plate as his girlfriend Jerry Hall, then guitarist Keith Richards looks like a survivor--one whose primordial power chords and death's-head demeanor have remained closest to the classic Stones anarchic spirit. His enjoyment seems genuine, his energy honest, and he looks like the kind of rock star who still tears his own T-shirts. Richard's chunky, unfinished attacks and Jagger's vocals are the only details one can pick out clearly on the sound track. Forget the keyboards, forget most of Ernie Watts' saxophone, forget Bill Wyman's bass, forget most of the bottom to Charlie Watts' drums. And look for saxophonist Bobby Keyes, who is listed as a member of the band but failed to appear on the print that I screened.

Then there's guitarist Ron Wood, who after seven years still can't seem to decide if he's in the Faces or the Stones. He is Richards' eerie shadow, moving like him, dangling beer bottles and cigarettes from a slovenly mouth. His supportive-dueling guitar sound is annoyingly buried, as if he and Richards were having a gun battle but only Richards had bullets. What's worse is that most of the band seems bored, making many of the 25 songs perfunctory in their delivery. Wyman, whose scowls once seemed genuine but now seem utterly benign, looks as if somebody forgot to plug him in, while Watts seems to be contemplating phoning in his part for the next tour.

Technically, the film is a mixed bag. On the plus side, the outdoor concert (at Sun Devil Stadium in Arizona) provides cinematographer Caleb Deschanel with the natural daylight and clear air that he used superbly in "The Black Stallion" and "Being There." Deschanel elicits a vibrancy from this footage, though with their cordless guitars and microphones, the Stones seem lost on the huge stage. When the action moves inside to the Brendan Byrne Arena in New Jersey, faces become pallid and the invigorating expansiveness of the Arizona concert is lost.

The sound is better inside than outside, but sound engineer Bob Clearmountain is as unable to draw sharp aural images for the film as he was for the concert album released last year. The cross-cutting from one concert to another seems ill-advised, a too-late effort at dizzying techniques. And the Maysles Brothers were more intimate and revealing using a single camera than Ashby and his crew are using 20. Ashby is obviously not trying for a documentary--there are only the briefest of backstage scenes, and no interviews with the group or its fans. He opts, instead, for an uncluttered concert film. In the process of being straightforward and up-front, he ends up being unimaginative.

The only clever segments are a high-speed montage of the concert stage being built and part of the concert that follows--(Dick Clark did it for Rod Stewart years ago) and some intercutting of a current version and a 14-year-old TV performance of "Time Is On My Side" (an effect ruined by some ill-advised social commentary). Oddly, there's hardly any difference between the two versions, or in Jagger's moves. There's a quick headline from an ancient rock paper: "Is Jagger Finished as a Rock Idol?"

"No, no, no," he sings emphatically, " 'cause time is on my side."

Of course, the appeal of this 20th anniversary tour, which grossed $40 million in ticket sales alone, was the suggestion that "this could be the last time."

"Let's Spend the Night Together" is a disappointing souvenir, at best a sweet substitute for the many who couldn't catch the Stones live. The Stones' status has always excused their shortcomings, so this film won't shake the believers. But it won't convince the skeptics, either.