A 'family gathering' for Alvin Ailey's 25th

The Alvin Ailey American Dance Theater is probably one of the most popular modern-dance companies anywhere, and it's easy to see why. Its dancers are, above all, performers. They play to the audience with a sincerity that completely cancels out any pejorative notions about playing up to an audience.

The Ailey dancers are genuinely warm. They're also hot with energy. Even the greatest companies have off-nights, but the Ailey never goes at anything less than full blast.

The Ailey troupe has also endured - for 25 years to be exact. Like the vitality in its performances, that ought not to be taken for granted in a country whose relationship with the arts has always been cautious.

Well, the Ailey group certainly isn't taking its survival record as a matter of course. To celebrate its quarter-century it threw a smashing gala at the City Center - where it performed for over a week before embarking on a ''25th Anniversary Tour'' (itinerary given at end of article).

Although galas are usually single events, this one had many earmarks of regular Ailey evenings. The most beloved dances of the repertory, such as ''Revelations,'' ''Blues Suite,'' and ''Cry,'' were performed, and, as always, the audience responded not only to the joy of these pieces but also in recognition of the honored place they hold in American dance.

Ailey performances always have a family-gathering atmosphere about them; for the gala, the family was extended back to all the alumni of the last quarter-century. It was marvelous to see the old-timers perform in tandem with the current company, and when the two - no, make that three - generations rollicked through the finale of ''Revelations,'' I bet even Scrooge gave a cheer.

Sentiment has always been the company's trump card, and in certain ways it's had to be. Except for a handful of dances of enduring vitality, the repertory is fitful. At best, it serves the vitality of the dancers and showcases the current stars, such as the voluptuous Donna Wood or the puckish Keith McDaniel. At worse , it reflects the pattern of the fashion industry. Each season, a new line must be produced without necessarily advancing new ideas.

The Ailey's new line for this year is the old line, done up with new music and decor. Decor, in fact, is the strongest element in one of the premieres, ''Blueshift.'' Created by the distinguished American painter Romare Bearden, the decor is a cityscape divided into six panels. The panels do so much moving up and down that ''Blueshift'' is best described as a dance for paintings. Yet the Bearden dominance also happens by default. Talley Beatty couldn't think of anything for the dancers to do but rush on and off with a breakneck speed. ''Blueshift's'' speed is an act of choreographic desperation.

Ailey's new ''Isba'' is choreographic fatigue. Ailey meant to work a touch of Turkish casbah atmosphere into the dance. One sees the attempt in the whirling-dervish motifs and rippling arm movements. One also sees Ailey wearying of the stylistic effort and throwing in all the old cliches with which the weakest part of the repertory is studded.

The full repertory, from the glorious on down, is in Princeton, N.J., through Sunday. From February through April the company is on a national tour. Early dates are Berkeley, Calif. (Feb. 6-12); Los Angeles (Feb. 13-19); Anchorage, Alaska, and Seattle (Feb. 20-26), and Tempe, Ariz. (Feb. 27-March 4).