At first I thought they did that to keep Darren around longer, but since Lea and Colfer are still fulltime regulars, it wasn't necessary.
I guess they wanted someone with a big fanbase on the McKinley side, but I agree, it was Glee at its most contrived.
I liked to imagine there was gonna be some running plotline about Kurt and Blaine growing as individuals during their separation, with Kurt learning to be less self-absorbed and Blaine learning to live for himself instead of making his existence revolve around Kurt, so they could reunite as a more balanced relationship and stronger more mature individuals, but since they're making out in a car the episode after next and singing Come What May the episode after that, it doesn't seem like we're getting it.