There's a difference between copying what one CCG does and implementing it compared to using a mechanic found in one CCG and adapting/converting it to fit in Elements. The only reason I was somewhat against it was it was too similar to a common feature in MtG. As such, if it had been incorporated into Elements, people would begin to akin Elements to MtG more and more, especially if other MtG mechanics found their way into Elements.
But, that's not to say looking at a mulligan rule from MtG, or any other CCG, and modifying it to work in Elements is not a bad idea or frowned upon. Only that any additions that are derived from other games should be changed so that, though they may hold similarities, they are unique enough to Elements that Elements can claim ownership over it.