Donald X was pretty insistent about all the when-trash cards being cards that can't trash (except Rats which can't trash other Rats) because he doesn't like self comboing cards (although he designed Minion), which is interesting since with trade tokens by necessity the cards must both get trade tokens and use them (unlike coin tokens which you can always use).
For what it's worth I don't think that's any problem at all, but I'm now thinking Craftsman should be the only card that gets multiple trade tokens easily, just to really emphasise that they are primarily the triggers for the one time effect of cards, and occasionally you might get to swap one for the other.
Ignore my previous post? Perhaps.
I think you're reading too much into it. What he actually said was this:
I wasn't specifically avoiding "has a when-trashed ability, also has a way to trash itself," but it's only natural that there isn't one of those. It makes the whole thing less interesting. There is a classic thing they sometimes do in Magic, where they put both pieces of a combo on the same card. It's just much less fun than when you piece the combo together yourself.
I'm going to ramble a little about self-synergizing cards, because there's a whole spectrum there.
Self-synergy is neither inherently desirable nor undesirable. It's good to have cards that you can load up on, because the alternative is usually loading up on Silver and Gold, which many players find dull.
Laboratory is a perfect example, but I don't think most players think of it as having "self-synergy".
The cards that most players think of as having "self-synergy" are those that have two separate abilities that combo with each other. Either the abilities are different options in a "choose one" list or one is on-play and the other is not, but they can interact with each other. Of the published cards,
Minion and
Governor are the big ones that people talk about. Minion's mulligan ability wants non-terminal, Coin-generating Actions in your deck, a role which Minion can also fill. Governor can remodel a card, so it wants $6 cards that can be turned into Provinces. Governor can also gain Gold (and increase your handsize to get Golds and Governors together).
Graverobber is a less-cited example of this mechanic. One of its abilities wants to gain $5 Action cards in the trash and the other wants to trash them (and Graverobber itself costs $5).
Donald is saying that it's more interesting when the two parts of the combo are on two different cards, but that only works when each of those parts interacts with enough other cards. Minion and Graverobber have a self-combo because they'd be too narrow without it. Minion got the +$2 option because the mulligan option was dead on too many boards; non-terminal, Coin-generating Actions aren't that common, partly because many of them are more elegant as Treasures. Graverobber's main concept is fishing stuff out of the trash; the remodel option is necessary because there aren't enough other cards that reliably put stuff in the trash that's good enough to fish out. I have no idea whether Governor's self-synergy is intentional. I'm guessing it's mostly happenstance.
On the flip side, some cards need a combo, but the other combo pieced was judged to be common enough that they didn't need to combo with themselves.
Rats is the big example. It's almost always useless without another trasher, but where Graverobber needs a partner that specifically trashes very good cards in order to be worthwhile, Rats can be at least somewhat useful with almost any trasher at all. A lot of sets have cards like this.
Farmland is probably closest to Rats on the spectrum. Even though tactically there are often reasons to buy it, it's really not offering much in terms of strategy and gameplay unless it has some other piece that combos with it (Border Village, Crossroads, etc.).
Some cards have self-synergy as their entire concept; you need multiple copies for them to do their thing.
Treasure Map and
Fool's Gold are examples of this. Sure, Fool's Gold also has its Reaction, but usually you aren't buying it just for that.
So, Enterprise.
First of all, I definitely don't perceive
Jubilee,
Guide,
Terrace, or
Exchange to have "self-synergy". Without the ability to get Trade tokens on-gain, they aren't just narrow, they simply don't work at all.
Craftsman self-combos in the same way that Silver self-combos. Actually, it's more like how Graverobber self-combos. One way to think about Craftsman is essentially that you're building a separate, parallel economy. A Craftsman is sort of a slightly-more-valuable Silver for $4. One Craftsman gets you a card costing up to $3, two get you a card costing up to $5, etc. The reason this is OK is that it doesn't combine with your other income (and your deck starts with 7 Coppers). How much you want to invest in that parallel economy should vary from board to board. As you get more of them, they do their thing faster. So if you're using them mostly to gain $5 cards, one Craftsman gets you a $5 card every other shuffle, whereas two will get you one every shuffle. Similarly, you can use the same Graverobber to trash a card and then gain it back, but it's slower than if you had two.
Craftsman costs $4 (and is balanced around that cost) to keep you from effectively rushing them. It takes two plays of a Craftsman to get another Craftsman, and at that point you could get a (probably more powerful) $5 card.
I don't plan to make another card like Craftsman (that both gains and uses tokens on-play), and I probably won't. But if I come up with another good one, I won't reject it offhand.
Redistrict is costed specifically to prevent trashing a Redistrict to gain another one. You can rush
Mill Towns really effectively in the early game, but by doing so you are digging yourself into an economic hole that you probably won't climb out of before the game ends.
Refurbish definitely has a self-combo, but like Graverobber it's necessary for power and fun purposes. Without its ability to turn other cards into Silver, your options are either use a Silver-flooder (usually not available) or spend most of your buys on Silver (boring).
Conscripts totally self-synergizes, but that's all part of its Barracks synergy. It's also player interaction in games with more than 2 players, so there you go.
Floodgate is the set's Rats, where its primary purpose is to combo with other cards. Hehe,
Committee has a self-nombo, or at least that's the way I see it. Buy two or more Committees; Committees reveal each other; I flood your deck with Committees! Better have a lot of villages, sucker!