I'd lock it to the rack outside and not worry about the rain. If you wipe it down upon return to your home and keep it lubricated, it should be fine. The advantage of getting a cover for it is that folks won't know it's your "good" bike. The disadvantage is that some might assume only expensive bikes are cared for in this manner. Oh, two other thoughts - having it under a cover adds just one more step to what someone has to go through to steal it; that might be a bit of a deterrent. And, by locking it to the rack, you can leave the lock locked to the rack and not have to transport it everyday. It is tough when we're worried about our newest "love".