The deluge of card games being released (I think primarily on the coattails of the success of Dominion), as well as the consideration of using cards in Franchise, sparked some thinking as to when it makes sense to use cards in a board game.
Even with pure card games, one could argue that tiles could just as easily serve that purpose. For example, Rummikub could just as easily have been a pure card game. I'm assuming that the production costs for tiles is higher than cards. In addition, the use of tiles necessitates that each player have a rack, which additionally increases production costs.
So why use tiles when the game would have been equally functional and cheaper with cards? The size of a standard card would have made holding 14+ cards quite unwieldy. Additionally, melding dozens and dozens of cards would take up quite a bit of tablespace (even though this could be mitigated through card design that allows for overlapping cards). In the end, tiles make sense from an interface design perspective.
Clearly, many cards games are just that because cards provide several interface advantages. Quite a bit of information can be displayed on a single card. The ability to hold cards in one hand allows for the information on the cards to be viewed quickly by the player holding the cards, while also keeping them hidden from his opponents. A stack of cards is easily randomized through various shuffling methods. A draw deck is an easy way to organize a pool of card from which a player can quick gain additional cards.
Sometimes the consideration goes beyond cost or interface design. In Ra, players draw tiles from a bag to place on a track to bid on. When you win the tiles, they remain face-up in front of you. Not only would cards work fine for this scenario, but I would argue that they would actually speed up game play. Instead of a bag of tiles, you have a face-down draw deck. The auction board might need to be a little bigger, but it could certainly be re-designed to accommodate cards over tiles. Perhaps this alone made it an interface desicion to use tiles over cards, but part of me believes that there was some aesthetic reason as well. There is a certain amount of tactile pleasure that people get from playing board games, and tiles certainly have a more substantial feel to them than cards.
As a final thought, I want to compare the use of resources cards in The Settlers of Catan to the resource components in Homesteaders. In both cases, resources gained remain hidden. Settlers uses cards to keep your resources hidden, while Homesteaders utilizes tokens that you place behind a screen. The biggest difference between the two is that in Settlers, there is no real advantage in knowing how few resources a player holds. However, knowing that another player is broke in Homesteaders can significantly change your approach to an auction. It should be noted that in Homesteaders you generally hold on to significantly more tokens (including resources, silver, trade chits, and victory chits), which also makes cards less viable from an interface perspective.
In the end, it appears that the decision to use cards is primarily an interface decision. However, it's important to consider how other components can be used as an alternative (or vice versa) when designing a game. Asking yourself, "What if these were/weren't cards?" may open alternate uses or streamlined gameplay that you may not have previously considered.