You know, it's not only the legendaries as well. Castform also has form changes. But I do believe that this generation was the one that introduced form changes. Black and White just got excessive with it.
Eh, it's good, but I don't like it too much. It feels so tedious, and the path you take to get from each city and badge in order starts to get all jumbled around the 4-5 Gyms and 7-8 Gyms. The first time I needed to look up a guide to find where to go for story.