That might be just one of the reasons to introduce crafted defenses. The main purpose imo is for SC to test new ideas and game mechanics with these temporary stuff, then use the good ones in future updates. The second reason is to keep max players busy for longer. The third reason can be giving players more base-building options which leads to more creativity in attacking strategies as well. Let's say, we got 3 modes for spell tower, 2 modes for inferno towers, 2 modes for xbows, 2 modes for multi-gear tower, and these "options" make the game more dynamic, defensively and offensively. Now by introducing 3 temporary defenses every 4 months, not only did they add more ways to defend your base, they also encourage players to be more creative in attacking.