I'm babbling about trading card games again. I've covered this before, but not for a long while.
Whoever decided it would be a good idea to limit the number of cards allowed in a a deck was a moron. (I actually know who thought it was a good idea, back in the early Magic days, and he wasn't a moron, but...)
Marketing Disaster - Successful TCGs have a uncanny power to drive repeat sales. Buy, buy, buy! It's a sales and marketing dream. Of course, if each player can only use four copies of each card, that sets a cap on how many cards are useful to a player. You've just told a customer that they shouldn't buy any more. It's like Coke making a soft-drink that stopped tasting good and quenching thirst after the fourth can...
Value Disaster - TCGs always have rares (which I also don't approve of, generally) which means to get four of a particular rare by openening booster packs, you normally end up with 30-40 copies of every common. By telling the player they can't use that many copies, the seller is effectively devaluing those commons to an amount close to ZERO. Players have figured this out, and the TCG market has suffered. Companies have not, and the TCG market has suffered.
Game Balance Disaster - The reason cards are limited to 1, 4, 6, or whatever, is because game-balance is put a risk if too many copies of a "Bad" card are released into the pool. I suggest making better games, where this is accounted for from conception to finished product. Unlike the previous two points, this is easier said than done. NOT IMPOSSIBLE.
My point to future TCG publishers. Stop trying to squeeze players by charging them for cards you know...FOR A FACT....they don't want or need, so they'll get those precious rares. Make ALL the cards useful ALL the time. Attached to a quality game, a sales goldmine waiting to happen.