Ohio State and Wisconsin played a year or two ago as well, in Lambeau Field. It was a huge success. I'm sure success like that one, and the one in Spartan Stadium, contributed (and will continue to do so) to the NHL and others having events like this.
I agree that with basketball, it was a gimmick. But with hockey, with so many hockey fans used to seeing youth and other levels of hockey played outdoors, it seems a more natural extension to me.