Needless.
Once in a while the addition of songs can enhance the experience: PASSION is actually better as a musical than it is in Ettore Scoala's film. And the score of LIGHT IN THE PIAZZA makes the material richer.
But in general, a lazy way to do a show. It shows how desperate creators are and the total lack of imagination on Broadway today. Take a hit movie, stick some second rate songs in it and get a 2 year run. FULL MONTY a prime example...what was gained by adding music? Nothing.
Even lazier - take a hit movie musical and put it on stage. Now 42ND STREET worked because it brought its own unique theatre experience to the material but SINGIN' IN THE RAIN, 7 BRIDES FOR 7 BROS, HIGH SOCIETY and MEET ME IN ST LOUIS were all deserved flops. None of them did more than try and recreate a movie on stage. LION KING offers a unique visual experience that is very exciting though the book and score are undernourished.
That is why successful ORIGINAL musicals (SPELLING BEE, AVE Q, URINETOWN etc) are to be prized.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com