The fact that Copper junking is an option even with Coppers in the discard pile makes it much weaker when it probably does not need to be. Even if the top decking was mandatory the Copper junking would be nearly ineffectual (i.e. only when a player is at the start of a shuffle) unless there is really fast trashing in the Kingdom.
I'm confused. Could you say that again in a different way?
When a player has Coppers in his discard pile, he now has a choice: gain a Copper or top deck a Copper. Attacks that give players choices are naturally weak because, even if both effects hurt, the player will always choose the option that hurts him less.
Top-decking the Copper will likely be the superior option to gaining a Copper unless you know an immediate reason you don't want a Copper on top of your deck (or you want Coppers, but then no-one is buying Magistrate/Bookkeepers anyway). Worst case scenario would be a four-player game where each player has more than one Magistrate: Then gaining a Copper or two might be worth it just to eventually get to shuffle your deck--but even then I'm not sure I'd mind so many $5 turns. Assuming top decking a Copper is usually preferable, the option to gain a Copper will now only hit when it is either (1) good for the player being hit or (2) at the start of a shuffle so there is no option.
True enough, though none of those provide +Buys either.
I don't think "It's never been done before" is a good reason to not do something on a Dominion card. In fact it's a point in the thing's favor. There may be other good reasons not to do the thing, but that isn't one of them.
In this particular case, I could see +1 Buy being a poor match for a junker. But I'm hoping this will primarily be a bureaucrat-like attack, and that seems fine. You could argue that it's bad that a cheap source of +Buy (which you might buy for that reason alone) also happens to be an Attack. You wanted the +Buy but now you're also attacking the other players. That might be the case, though it hasn't stopped me from buying Taxman for the Treasure upgrades or Soothsayer for the Gold gaining. Or any of the Smithy attacks for the +3 Cards. Anyway we'll see how it plays out in practice.
I was really thinking something along the lines of what GendoIkari said, but could not quite formulate it. It struck me as odd, but GendoIkari hit the nail on the head regarding its anti-synergy, but I will see if I can explain some other knee-jerk reactions to it.
+Cards and +$ are rather simple effects that combine fine with junking because they are simple, low-power abilities that are always useful in a deck, but +Actions and +Buys are significantly more board dependent and are often the reason that cards are bought.
Ambassador,
Soothsayer, and
Torturer are probably the only junkers I buy for both their effect for me and their attack on other players, and I find those cards to be rather degenerate and un-fun. What I'm saying is: Most cards that junk aim to junk and do not do much else and the cards that do are unreasonably powerful in a way that I am not sure is healthy for the game. It is probably okay because Copper junking is weak.
The best argument I can see for Bookkeeper having a +Buy is that it is a bonus that does not work well in multiples to reduce the card's efficiency when stacked (as its Copper top decking gets painful enough when repeated).
