"Penetration" is no the importart part. "Presence" is. "Competitiveness" is. By being lax on online games, they've basically been sending forth the attitude that they really don't care what Sony or Microsoft are up to--that they don't care how immensely cool some of their games would have been with online play or how many gamers want it--they just "can't be bothered." They already HAVE the equipment to go online, and it does get used with a few games, so I imagine many people are crying "WHY IN HELL DIDN'T THEY TAKE AN EXTRA WEEK TO STICK ONLINE MULTIPLAYER IN MARIO KART?"
Most people don't know how long it takes, you see, but are quite willing to think that anyway.
Embracing multiplayer even in just key games where it would have the most effect would be all they really needed to have gamers sit up and take notice. It increases the longevity of the games, the visability and competitiveness of their console, the interest in new sequels with it added... Considering just how much cash Nintendo is sitting on top of, how hard would it have been to add online multiplayer to a dozen key titles even just to PS2 levels? To toss a bit more support to certainly publishing houses to get their games online as well? The answer is "none at all."
Also, most importantly, it builds a foundation and gives them the experience to make online connectivity BETTER next generation, but whereas Microsoft has built a recognizable service and Sony has gotten its hands into games all over the spectrum, Nintendo has shrugged its shoulders. They automatically start next generation in a lower gear.
If "penetration" were all that really mattered, shouldn't they have given up on the GameCube in general? I mean it probably only amounts to 15% or less of the current console generation, right?
There's no two ways I can go about thinking their lack of even MINIMAL online support is anything but a mistake. It may not be "as profitable as they'd like" but it IS profitable, it is nothing but an enhancement for their console, their competition has it, and it IS the future of gaming. Why limit themselves?