I have both, they have almost no overlap in my mind. Yes,the iPad can game. Yes, the 3DS can go online, but each one does certain things better then the other.
Its totally worth having both in my mind...I love gaming, and while the iPad is fun for a quick game here or there, its not a dedicated gaming machine like the 3DS. And while I could surf the web if I had too on the 3DS, I'm much prefer the iPhone or iPad.
To the person who said this was a stupid combo, you are clearly not a gamer, thats fine, but no need to bash someone who is.
As for iPhone with a tethering app plus iPad with Wi-Fi, thats a good solution for many people, but for other they might not need that. Take me for explain, I had Wi-Fi pretty much all over my campus, which is where I go to class and my job is. That gets rid of my need to tether, plus saves me the trouble of jailbreaking. Also you suggestion does nothing to help the gamer the OP seems to have inside him or her.
Long story short, there is no one single combo that work for everyone, and there is no need to suggest one combo is terrible without first knowing that a user wants and what their situation is.