The main cons for you are they will be using a service you are paying for, your bandwidth will be reduced (slower surfing and upload / download times) and your computer security will be compromised.
The pros are all for them, free broadband, free to surf and do whatever they like online possibly illegal stuff (which will land at your door as it's you who is registered with the ISP not them).
So basically don't do it as it's not secure and you will be worse off.