I agree card games can be a decent training mechanism, but I think the efficacy increases with the games complexity. I played MTG in my youth and a number of other card games like L5R. In actuality I think MTG is one of the least complex card games in the market, lending to its wide popularity. Certainly it could be argued to be more complex in some ways then poker, but less then other games. At some point complexity makes a game overly convoluted or turns off more casual players if it reduces the ability for an average person to understand its basic facets (one advantage of games like chess/go is easily understandable basic rules, yet very complex and worthwhile strategical depth).
I tend to think any game can be construed as a means to train your thinking/brain, and in this case more specifically rationality, depending on the content. I also think things with a time metric allow more complex decision making due to the inherent time/thinking management issues. However, depending on the specific details of how our brain functions with respect to solving problems in specific games, it's hard to say what exactly is being improved unless tested.
An interesting fact is that I use to be an extremely competitive starcraft player, and some of the skills used in this game have a good correlate with games such as poker. A number of my fellow professional starcraft players moved on to poker where they now play for a living.