There's this factor going on here, that, players who are really really good at the game find uses for weak cards, and intermediate players don't. It's because if you don't have that super epic understanding of the game, [addbuyrule: don't buy Counting House] is going to work better than [addbuyrule: buy Counting House only if 3 +buy sources], or whatever inappropriate conditional way of picking up a card the intermediate player comes up with. It takes a high level understanding of the game to improve on the "don't get it" buy rule.
As someone once posted on the forums shortly after Hinterlands came out: "The best way to use Mandarin is to not buy it at all until you hit 30. Then when you hit 30, only then do you look at Mandarin, think about how you might perhaps use it on that board, and then don't touch it until you hit 40"
So, when you're making a determination about whether a card is truly too weak, you should defer to the best player in the room, really.
Most cards that degrade the quality of dominion really are the high-power outliers, not the low-power outliers.
If you wanted to make changes regarding the weaker cards, it'd be better to nerf high power cards and let them get stronger as a result, or to experiment with different rules regarding what gets included in the kingdom and what doesn't. If you want to see Counting House induce more variety in the game, then give it a 30% chance of appearing in addition to the other 10 cards, Counting House is good in like 3% of the games it appears in so you'll get your 1% shakeup. I'm actually quite interested in the prospect of a Dominion variant that includes nothing but weakish cards, and a large number of them, I think it would create lots of decision intensity. Stuff like Bureaucrat, Harem, and such, and unpopular engine cards like Walled Village and Council Room.