[citation][nom]SAL-e[/nom]It is a big problem. If I can get access to TW router I will own your network in no time.1. I will change your DNS settings and redirect all your traffic to proxy that I control.2. I will monitor your traffic and collect all your passwords quite easy.3. I can perform "men in the middle" attack. None of the security protocols will protect you if I can control your TW router.[/citation]
First of all, if you re-route DNS, the change will show up in the logs of the secondary router which you don't control. The secondary router will prevent you from directly infiltrating my network from the compromised TW router. So even if I wasn't aware of the gaping security hole in the TW router, I would know something is up. You would have control of my network traffic for maybe a few minutes, and the only thing you would be seeing with your packet sniffer is ping and network traffic tests as I try to figure out the sudden increase in latency of my Internet traffic. It wouldn't take long to figure out that there was a problem with the TW router, and then I would focus my attention there. Since I still have physical control of the hardware, I would perform a reset and then monitor incoming traffic to the TW router.
Anyway, that's what I would do. Someone less sophisticated would still benefit by putting another router between the TW router and their network because the second router prevents a hacker from COMPLETELY infiltrating their home network. All banking and credit card transactions use SSL and TLS encryption protocols, so you won't gain anything from there. I don't know if online games take the same precautions, so you might be able to hijack someone's World of Warcraft account.
Honestly, though, if your intention is to do any of this, your efforts would bear more fruit if you went war driving and attacked networks with unsecured wireless APs. If they don't secure their wireless routers, then chances are their computers are going to be under-protected as well. Or, another thing you could do is go to someplace like a university campus, or airport where there is free wi-fi and plenty of people using it. Setup your laptop to look like local AP, and then being your packet sniffing and MitM attacks.
As I'm sure you well know, no amount of network security will prevent the intrusion of a determined and skilled adversary. It's more like putting bars on the windows and doors of your house. Sure, a burglar could still get in if he really wanted to; but why waste the time when the neighbor's house has no bars, and... look at that, the back sliding door is unlocked. That's the point I was trying to make with my original post.