Alison appears in 2 of the films scenes, in the prologue (where, unlike the book, she only shares drugs with Sickboy as opposed to sleeping with him as well) and in the tragic scene involving Dawn (where she acts as a proxy for the character of Lesley).
It was and is simply a way of making the characters play a more economical role in the screen story as compared to what are often sprawling books with a multitude of overlapping characters. The film (and to be honest the book as well) seems a lot more interested in the guys than the ladies and as such the characters of Lesley and Allison were combined into one person.
This is done frequently throughout the film, such as the combination of Rab "Second Prize" McLaughlin and Danny 'Spud' Murphy (even the book refers to how similar they are and with both 'having never been seen in the same place' a joke is made regarding the possibility of them being the same person.) The most dramatic example may be the combination of Matty and Tommy, while as they both contract HIV in the novel it is Matty who dies of toxoplasmosis.
This is often done in screen adaptions of books that have a lot of characters - one of the more classic examples of this in the most contemporary of pieces can be seen in The Lord of the Rings: The Fellowship of the Ring where the character of Glorfindel was not included, with the film-makers choosing instead to expand Arwens role to cover events Glorfindel originally dealt with in the book.