It is often a design decision to normalize data into multiple lookup tables and create reference fields for that data. Do take note and caution when doing this that there are limits on the number of Indexes and Columns that can be created on a single table (and if you are extending a table like Task then realize that some indexes and columns are already being used up and or get used up in the flattening process).
Reference fields are indexed and can push you closer to the upper limit of 64 indexes (indices?).
What I am trying to say is that it is a good design practice to control your reference fields.
Introduction to Fields - ServiceNow Wiki
Tables and Columns Module - ServiceNow Wik
