Most reasons people mention why Nintendo shouldn't aim to be the second console are mostly emotional and ego.
They're going to launch a year after 360, and possibly head to head with PS3 if that is delayed (which some developers seem to think will happen). They're not going to be number one, whether they shot for it or not. Their relevance in the console market is questionable. Why fight a battle you're not going to win? Even from a business standpoint it's simply inefficient.
The train is already on the track. They're aiming to be different. The fate was sealed with the controller. Even though they're all making games, Nintendo's track is going in a different direction.
Didn't Reggie's speech mention that there is a 25% platform crossover? That's in a market where they are all essentially competing on the same level. GameCube somewhat competed as a second console, at least in price, but that wasn't until the price dropped later in the cycle and the system's fate was already determined.
Nintendo is banking on 360 and PS3 being too similar for most people to want/need both. If their assumption is correct, the crossover potential is Nintendo's to own.
If Nintendo proactively markets Revolution as a secondary alternative (whatever terminology they use), raises the crossover penetration modestly (even just to 30%), keeps the price low, continues to sell to their built-in fan base, AND bring in the new players, they have a fighting chance of increasing their market share while the other guys keep dogging it out. They'd probably still end up in 3rd place, but with more marketshare and mindshare. Hopefully they'll have sold the concept of the controller, which helps them in the fight of 2011 if the others copy it.
A lot of steps there, but it's possible.
Re: 1st year. As I mentioned I think the 2nd year will be just as important as the first, considering the distinct part of Nintendo's strategy to grow the market with new players. The first year will be all about early adopters and the built-in fan base. They're the ones that would have bought it anyway. The newbies aren't going to pick it up fresh off the assembly line. Those buyers need time. The crossover buyers need time too. Year 2 will be about riding momentum that year 1 begins to execute.
Of course, EVERYTHING hinges on this darn controller. It's either genius or insane.