>Alltrim(Last)+space(1)+Alltrim(First)

This is not sufficient if there are spaces in the first and/or last names. For example, you would want Jim Van to appear before Jim Van Deusen. Yet the index expression above would yield

VAN DEUSEN JIM
VAN JIM

The point of not using AllTrim() in index expressions is to make sure the first and last names are evaluated separately with regard to sort order: i.e., to sort by last name, then by first name if there's a tie.