Completely agree about the diminishing returns on Nintendo Direct. I obviously don't have market data to back it up, but the way Nintendo handles them sometimes is quite baffling. I have no reason to believe people who do not own are Wii U are, in droves, watching Wii U related Nintendo Directs to see if they want the system. They already don't care. The people who watch already own the platform, and are interested in the games.
Granted, the reasoning behind Nintendo Directs could be that they're just a fancy pants version of a press release. Yes, they appeal almost exclusively to platform owners, but people will still report the news, tell others, post threads, and so on. They might be thinking of Nintendo Direct as a viral kind of thing.
And honestly, as long as they're holding press related events at E3 (which it seems they are), I guess they're filling the purpose of the single conference incrementally.