The vast majority of OBi users have wireless routers. Even if you don't normally use the wireless function, it's useful e.g. to allow a guest in your home to connect to the Internet, or to temporarily use your computer in another room. You should be sure that the wireless function is set up securely, so neighbors don't freeload off your connection, which could degrade the voice quality of your OBi, and possibly get you in trouble if they do illegal downloads. If you are really paranoid, disable the wireless function when you are not actually using it.
If you are on a very tight budget and don't foresee a need for wireless, you can probably find a used wired router at a very low price. One of your friends may even have an old one to give you. Also, a few people are disturbed by the radiation from routers and other wireless devices. Otherwise, get one with wireless.
