Well first off, there ARE cards that can attack other creatures, and ones that can defend from other creatures. So I'm not exactly sure what you are suggesting... that they ALL should?
Secondly, being "rewarded" for showing other people the game is, to an extent, pointless. VERY few people I know even look at their invitations on Facebook and their e-mail to things similar to this. So giving someone free stuff for barely doing anything is extremely stupid in my mind. Even if there was the "spin the wheel for a card" after you won, there would be like 100 cards on that wheel so you would basically never win anything. Even if you did that would only be like 30 coins which is an extremely pointless amount in the big picture.
Thirdly, putting in that extremely pointless feature really should go way in the back of the developer's priorities. There are much bigger issues to be addressed then recruiting 1 person per 100 cards he hands out for referrals. The best way to get someone into a game you are playing would be to show it to them or tell them in person or over the phone/ventrilo like programs. And if you did that you wouldn't even be acknowledged by that "reward system" that you suggested. And there are already 78,000 accounts (I think thats what somebody said previously.) I've also seen this on other sites ( I found this on kongregate.com myself.) So he is getting the game around as it is.
Edit: A referral system IS A spam system. You try to refer as many people at once to get rewards like you suggested. I see it all the time on (I used this example a lot) Facebook app's. Nobody even bothers to LOOK at what app people wanted you to go to. Half the time they don't even want you to join, they just want their reward.

