This drives me crazy, too. However, Andrews, Askey and Roy give their reasons for preferring the Legendre definition in their book on special functions. See http://books.google.com/books?id=kGshpCa3eYwC&lpg=PP1&dq=andrews%20askey%20roy&pg=PA6#v=onepage&q=legendre&f=false

I'm not sure exactly what they are referring to in their section 1.10, but this may have something to do with it. http://books.google.com/books?id=kGshpCa3eYwC&lpg=PP1&dq=andrews%20askey%20roy&pg=PA39#v=onepage&q=jacobi&f=false

This drives me crazy, too. However, Andrews, Askey and Roy give their reasons for preferring the Legendre definition in their book on special functions. Click on the link to page 6, http://books.google.com/books?id=kGshpCa3eYwC&lpg=PP1&dq=andrews%20askey%20roy&pg=PA6#v=onepage&q=legendre&f=false

I'm not sure exactly what they are referring to in their section 1.10, but this may have something to do with it. Click on the link to page 39, http://books.google.com/books?id=kGshpCa3eYwC&lpg=PP1&dq=andrews%20askey%20roy&pg=PA39#v=onepage&q=jacobi&f=false