There's two problems with unique indexes on versioned data:
1) The new data is added to the Adds table, so the integrity constraint wouldn't
occur when needed
2) It's perfectly valid to have multiple unique values in the Adds table, so long
as only one of them is eventually posted to the business table.
There's also a performance cost associated with unique indexes in general, so if
you're not going to see a benefit from that cost, why pay it?
- V