With games like pontoon/21/Black-Jack it is possible to be in a consistent net-win upward path by understanding the odds of winning each hand. With games like Poker, bluffing is a big part of the game and thus is on the whole less predictable. With other games (e.g. Roulette), there is much more luck and randomness.Back to topic, I can support in-game money gambling as a novelty mini-game or something, like playing for caps in Fallout 2 and New Vegas. As here isn't a "luck" skill like as in those games I think it could be a credit sink and with luck, we all learn the valuable lesson that there is no way to win in those games.
True gambling in general though is a mug's game, and anyone that engages in it should only bet as much as they are prepared to lose (regardless of the nature of the currency being gambled).