Personally, I think vanilla half harem is fine. They can't all be the best card ever.
Also, concubine.
I think the problem is the cost. $3 is simply too much, but at $2 it would be strictly better than Estate. It is a card that, if you started the game with, you would want to trash out of your deck, just like Coppers and Estates. The only 2 times you should ever be willing to buy it are 1) times that you would otherwise buy Copper (pretty rare) and 2) times that you would otherwise buy an Estate (only towards the end of the game).
Sure if the game is close to over, and you have exactly $3 or $4 to spend, you choose it over Estate. But otherwise I'm just pretty sure this would be purchased far less often than Explorer, Scout, etc.