[vinaypai]

There are reasons to want your own modem, to avoid overpaying via rental fees, for example, but what security or privacy do you imagine you're getting by buying your own modem?

You're literally plugging it into their network and they can see everything that goes on the pipe whether they are on the LAN side of it or the Cable side.

There's a stronger privacy argument for using your own WiFi access point though.