It depends on how frequently "sometimes requested" is, and how big a datatype that extra column is. Since an index is a tree structure, adding that extra column as part of the index key means SQL will be able to store less record keys on all the index pages, so while the sometimes query with the extra key will perform better, the queries that only use the first key fields would have to do more I/O to scan the same number of record keys. With INCLUDE columns, that extra value is only stored in the leaf nodes of the tree.