You know, it's like giving an attack boost, with a situationally useful side effect of having a creature in play the whole time. The proper formula is the following: if the chance is n%, then it is equal to an attack bonus of ATK / ( 1 - n% ). For 50%, for example, it is equal to having a 6/0 BL and a 4.5/0 Spark. Not really broken, is it?