Yes, I agree that it’s immaturity, that’s why it’s the cynical side of me that believes it (and the optimistic side of me which says that even when it’s a factor, it’s not the only factor).
As far as wondering why the benefit is so large as to play such a central role, I think the feeling is that it does set the tone for their lifetime together, both regarding a Torahdik’e home (your “wrong” reason), and her commitment to supporting his learning throughout their marriage.
There’s another, probably more correct reason, although, similar to your “right” reason, I don’t know if it’s actually part of the thought process. http://www.theyeshivaworld.com/coffeeroom/topic/working-and-learning#post-425912