For all these folks, matching theme with mechanics helps make the game more compelling and exciting. While it's true that a good mechanic can usually survive retheming (or even carry a game with little or no theme), theme helps make mechanics comprehensible, intuitive and easy to learn. What Eric Zimmerman, designer of Quantum, says about cards is true in general: "[a] card title and image serve as a kind of cognitive 'gateway' into understanding the card function. They help orient the player, making the rules component of the card easier to digest (and remember)." A tight match of theme and mechanics simplifies a complicated game, makes it easier to learn and retain the rules and helps keeping the game going smoothly. If you think about the adding of cards to your character as 'equipping' them, your mind will automatically makes assumptions that help the game: you can use it while others can't, you can't have too many things equipped at the same time, you can hand it over to someone who is nearby and so forth. At the same time, a game that violates thematic assumptions ("what? I can equip this but never drop it? How does that make sense?!") can be confusing, disorienting and very frustrating. Lastly, even most hardcore mechanic oriented players like a theme a little bit - it fills in the blanks of their superior wits in XXX, and gives a bit of meaning to what they did when they destroyed you just there.
For thematic oriented players and designers, who typically care more about the narrative of the game, there is sometimes a tendency to add more and more mechanics to accommodate the needs of the story. A game like Descent, that focuses on story and character development, ends up adding more and more rules as the game progresses. Each scenario has some special rules that are added in order to capture the situation of that specific mission - its goal, its circumstances and the twists and turns of the plot. Every piece of equipment and even every monster is a little rulebook which changes the game a bit to give you the feeling that your character is progressing on its narrative arc in the world of the game. This is a very challenging design work, because there's still a need to keep the mechanics balanced within the great variety of options. The strength of thematic games is that they let people experience the story more closely - and it's no coincidence that they are somewhat similar to role-playing games, as they aim for the same kind of experience. But they are weaker if their mechanics don't present a great challenge, are are ever-changing or just don't make any sense. A good thematic game has decisions that make sense even if you strip them down to their mechanical bare bones. A good example of that is Dead of Winter, where each crossroads card has a story that presents you with a thematic decision like 'should we accept more survivors to the colony'. At the same time, it presents you with a great strategic decision that can be considered mechanically - do I pay the costs of one extra food token per round in exchange for an extra action die, another pawn to move around and a random special ability from the deck?
So much about matching theme and mechanic, on which much has been said. I raise this old issue again not to give another answer to the question 'how should we balance the requirements of theme and mechanics in a game' but rather to discuss how the choices made in the balancing of theme and mechanics impact the politics of a game, or as I like to call it - the little social world created by the game. My previous post on political analysis, which explains that it focuses on the way players relate to each other within the game, might suggest that it is mainly concerned with mechanics. When I asked 'what can players do to each other in this game' I mostly gave examples of mechanics - they can take resources from each other or they can't, they get assigned special roles by the game or they don't. Does it matter what thematic explanation is given to describe the fact that you take a cube from somebody else's reserves?
I think it does, and an example might serve to explain. In Village, your meeples die as time passes. Presumably, they die of old age after a long and satisfying life as a carriage maker, traveler or monk. Mechanically speaking, time is a resource you can spend (and usually, though not always, avoid spending) to gain some other resources. The cost of spending time is that occasionally, specifically every 10 units you spend, you have to give up one of your pawns (aka meeples/workers/etc.), in a certain order. Technically, that's not very different than trading sheep for wood, but in Village the game goes to great length to persuade you that the meeple really did die in peace, literally laying them to rest in the Village's chronicle or the graves behind the church.
In any case, I think it seems to me that putting your meeple to sleep on the pretty board in front of you feels very different from trashing a card, returning it to the box, while the evil cannibal is literally gazing at you from the table, holding his hands above a bloody plate. And this difference makes sense - Village is a game about the euphemized and imagined peaceful life of medieval Europe, which some have compared to a nice cup of tea for good reason. That meeples can die in this game is pretty surprising, as it belongs to a genre of games that avoids conflict and really doesn't want to make you feel bad in your stomach as you play. That they can pull it off and still appeal to the Euro crowd is an impressive achievement, especially since it is sometimes in your favor to have your meeples die sooner rather than later and the game incentives you to control this process. This thematic incongruence which is successfully integrated into the game is what most reivewers cite as its main innovation. In contrast, Arctic Scavengers does not want to make you feel nice. The game description starts with the death of the 90% of the world's population and then adds, if you missed the point that the "The world of Arctic Scavengers is cold and brutal" (emphasis, I kid you not, is in the original). no wonder some people said it was going to make you feel scared, and they said it while pretending that people are shooting at them. Go figure.
How one understands a theme is open for interpretations and will vary from person to person. Of course, this doesn't mean that it's meaningless to have a discussion about it or point to some aspects that are not completely subjective. This is, of course, dangerous waters, but it's part of the mission of the political gamer.