> I suspect it would not be used as often as the primary key index
That doesn't matter because it's the creation of the index entry that matters, not how often it's used for lookup. The lookup cost is the same anyways.
> Since workloads commonly are interested in recently inserted rows
That's only true for very specific types of applications. There's nothing general about that.
Plenty of applications grab rows from all time, and there's nothing special about the most recent ones. The most recent might also be the least popular rows, since few things reference them.
That doesn't matter because it's the creation of the index entry that matters, not how often it's used for lookup. The lookup cost is the same anyways.