It's not that uncommon. It's a completely reliable solution to the problem of attributing sales and knowing how much each advertising channel generate individually in sales.

But taking into account that almost all jobs in advertising depend on keeping it "a mystery", it's no surprise that relatively few companies do it.

After all, it looks better if you tell your boss or your customer that they had 40 000 "impressions" thanks to your campaign, rather than 400 definite sales.

It is very convenient when you can point to others for moral absolution when the victims are invisible to you.