Lv. X cards are the new premiums that replaced EX cards. And there are some benefits and cons to playing Lv x to EX.
Cons:
Never a basic that you can play straight up. You have to wait at least a turn to level up.
Not always as powerful as an EX.
Not as many of them in sets, making them harder to pull.
Pros:
Not worth 2 prizes.
Many have been reprinted as tin promos, and easy to get if you don't go the booster route.