I've been thinking about that and I'm sure we can come up with something to keep them interested in using them. Tournaments can be won and lost over such things.

Some tournaments will use the Plus rule, some won't. The downside of the Plus rule is exactly what you were talking about - people are more likely to get away with not using higher level cards if the Plus rule is in effect. Tournaments for high level cards will probably not have the Plus rule in most cases purely to encourage people to use their most powerful cards.

But any other ideas would be welcome regarding this, such as "If you have a level 10 card and are entering a tournament for another level 10 card, you have to use the level 10 card" or something like that. That's just off the top of my head... I'm sure we can come up with other ideas!