I think questioning the player as to how the creature is being pinned is neccesary, especially with certain monsters, ie: Displacer Beast - large creature and pinnable, however- it also has the use of it's long tentacles. Unless the player has specified exactly how he/she has performed the pin, those tentacles could be quite effective for cancelling the power. In addition, shouldn't a strength check be required at the start of the players next turn to see if he/she is able to continue pinning the creature as well as an endurance check to see whether or not the character can withstand the strength of a monster trying to escape? If anything, a "save ends" should be attached to the power.
I've noticed a few powers in the PHB like this that seem kinda broken.
I've noticed a few powers in the PHB like this that seem kinda broken.