Heres an hypothetical situation: tell me what you think.
hacker, lets call him Tim, goes and gets a network scanner and scans the Telkom I.P. range for routers.
He finds one and cracks the password as you left it on admin/adminsitrator/the default password.
He then uses X-pass or something to get the ADSL line password.
Now you can manually define DNS in a router-fantastic. Tim creates his own DNS server with his own machine and uses the Telkom DNS as a reference. Now he can capture all your data and the victims internet will be slow but not stopped.
How? because a packet sniffer can capture all the data as it passes through his network card.
Now all this is going to use a lot of data so he enters in your own ADSL line into his router.
Now he starts to capture the data. He gets some encrypted data but since he has captured the negotiation process and all the keys/encryption process, he can decrypt the data. Also known as a replay attack.
Fantastic-he now has email addresses, banking details and anything else valuable and it hasn't even cost him any band width. It costs you twice the bandwidth AND you lose all your money...so you can't pay for the bandwidth anyway.
The end.
Now go and change your default username and password on your router.