Generally they work very well, better than audience category targeting. To work the best you have to set them up so.
If they are following you post purchase it often (but not always) means the marketing personal have not built a negative audience on a purchase confirmation flag to exclude this behaviour.
It may or may not matter depending on the ad pricing. Some ads you only pay on a click, others on views shown. Obviously the latter wastes more money while if click cost its not such a problem however if volume is great enough to effect your ads likelihood of being clicked it might effect how the ad network displays your campaign.
Basically ads are only as good as the product & implementation. And without trying to sound elitist many of the people setting up and running these campaigns are less technical than they should be and a bit useless at their job. In the same way there are amazing coders vs guys that do the bare minimum pasting other people's scripts as all they can do.
Odds are these campaigns are still making money overall on following the people that didn't initially convert so the companies are happy to keep them running as is.
TL:dr Most likely the ad manger has done a poor setup in remarketing.