Naomi,
NOLOCK is a sort of last resort item in my book... it typically means that there are so many other things wrong with the database/query design or a seriously underpowered server if you need to use that routinely to fix query performance.
>I remember comment of the previous developer regarding one of the SP (SQL Roles provider). He added NOLOCK hint and indicated that it was for performance reason. Perhaps he observed the same behavior as you do.