Its not about them getting your Answerbank details.
However if they can connect to the internet through your router then they could be doing all sorts of things (hacking, criminal activities etc) which YOU might get blamed for as it was YOUR router.
Also they could start downloading films or whatever over your broadband to THEIR computer, making your system slower, and maybe using up your Broadband limit if you have one.
Also if, for example, you were registered on Amazon (or eBay) they could hack in to your computer and find you Amazon / ebay Logon details.
They could then change the address to THEIR address and order a Rolls Royce or other expensive item using YOUR Credit Card details.
The first you would know about it was when you get your credit card bill.
A few years ago I used to work for a large computer company and they had wi fi all over the building.
Some employees would drive close to the building at the weekend and without even getting out their car they would connect to the wi fi and download all their mail.
Make sure you have good security on your wi fi.