(I'm also a little curious about Nintendo's reason for shipping a relatively underpowered system, and if it relates to what I understand was some weakness in Nintendo's share price back in '02. Perhaps that weakness forced them to invest less and so aim lower in power, or maybe they've been on that path for a while.)
Before the Wii and most Ds sales Nintendo was only worth around (from the top of my head) $4 Bil, compared to Sony being worth $30+ Bil and MS being worth everything.
How can a company worth a fraction of the competition compete with graphics that will set them back into a loss for every console sold.
Up to today MS Xbox team has not yet made up for the 4 billion the original Xbox lost them. So it wouldn't make sense for Nintendo to compete graphically down that road.
It's about money at the end of the day, Sony makes a loss per Ps3 sold, so did the X360, Nintendo makes a profit per Wii sold. So they can't loose.
You get what you pay for.
So it isn't underpowered.