Well, i had this doubt for some time now. Why do we need 2 notations for the same purpose? the prupose of collecting and anlysing customer requirements. Well, this link came to my rescue. There was a nice clarification to saying that Use case in text form is the presribed format though UML does have an equivalent diagram to depict the same. Satisfied not to use UML tools to draw simple diagrams for ages :)
Craig Larman too emphasizes the same point and goes on to say that drawing diagrams for long time without having the text version done is not good for the design.

Here is the pdf for Larman's justification http://www.craiglarman.com/book_applying_2nd/06-use%20cases.pdf