Right now it seems as if the Wii U's biggest issue is that it doesn't have any Nintendo system sellers yet, or at least not a sufficient number for somebody to buy it if they're on a tight budget. I personally have yet to buy a Wii U, because money is quite tight and I'm waiting until I'm "forced" to (enough Nintendo games are out for me to buy one.) I was thinking: how popular are Nintendo games? Does their popularity fluctuate? If so, can the popularity of Nintendo games decrease with less exposure? But that wouldn't happen with the existence of the 3DS and the exposure it gives Nintendo franchises through its larger base and high quality releases. If there are people who like these franchises on the 3DS, certainly they'll buy a Wii U as well, eventually. So the logical conclusion is that it is inevitable that with more games Wii U's sales will pick up, not just moderately, but significantly because it makes the fans of these franchises less resistant to buying a brand new console knowing that not only one of their favorite games is on the console, but multiple games they enjoyed very much on the 3DS (or priorly) can be found on the Wii U. 

So all in all, by supporting the 3DS, Nintendo is maintaining its [Nintendo's] significance for when the Wii U is buyable [to those who are more conservative with their spending.] If they didn't support the 3DS as they are, the franchises might lose their word-of-mouth and significance in modern memory  when they are actually released on the Wii U, and this would be quite devastating on all ends, with a weaker 3DS and a perpetually weak Wii U. 

Overall, the 3DS and Wii U support one another, and boost eachother's sales because they have slightly different audiences who enjoy the same franchises. 

P.S This thread was partly for those of you who think of the 3DS as a parasite of the Wii U by taking Nintendo's time and resources. 

I did consider the GBA and GameCube, but I figured that Nintendo had a stronger monopoly on handheld gaming and that partly contributed to the GBA's success without being anywhere near the system the DS and 3DS are: before mobile phone gaming, before PSP, etc, etc. One can compare the sales of Mario games

on the GBA

http://www.vgchartz.com/gamedb/?name=Mario+&publisher=&platform=GBA&genre=&minSales=0&results=200

and on the 3DS

http://www.vgchartz.com/gamedb/?name=Mario+&publisher=&platform=3DS&genre=&minSales=0&results=200

With a smaller install base and still plenty of time to sell, the 3DS games have been more successful. 

no, what kind of question is this?

I think the handheld and home console market are too distinct to see the success of the 3DS have a significant impact upon the Wii U. 

When it comes to home consoles, Nintendo seems to be mostly dead among teens and young adults. And without them, it's almost impossible to dominate the market. That crowd want pretty graphics, online functionalities, FPS, sports simulations, open world games, cinematic games. And definitely something that look mature even if it's not. In pretty much all those spheres, Nintendo and Mario is nowhere to be found.

Unless they pull a Wii among the traditionally non-gaming crowd or drastically change their overall strategy, I just don't see Nintendo's home consoles ever dominating the market again.