I don't have experience with a wireless ISP but I can answer some of your questions about why they want to sell you a modem. First, it assures them that you have a strong antenna and signal to connect with them. Second, some of their customers may have desktops without a wireless card. And third, it gives them a consistent interface to configure which eases customer support. However, if your computer can communicate directly with the ISP, then you shouldn't need the modem.