There are times when a single key column index can be useful, like for a unique constraint.
But for the most part, outside of the occasional super-critical query that needs to be tuned, single key column indexes either get used in super-confusing ways, or don’t get used at all and just sit around hurting your buffer pool and transaction log, and increasing the likelihood of lock escalation.
Read on for Erik’s full point. Sometimes that single-column non-clustered index really does do the trick—as in a unique key constraint, or a single column used in a really commonly-used
EXISTS clause—but it’s worth thinking about whether that one column is really all there is.