Eventually, even "fair-weather" riders will encounter rain. And unless you have some sort of waterproofs available, it's going to become a miserable ride home.
Waterproof clothing doesn't have to be expensive. A cheap pair of over-trousers and a lightweight showerproof windbreaker will do the job. These usually have elasticated cuffs and waist, a close-fitting neck, and no flaps. The advantage of this type of gear is that it can be rolled into a very small bundle and tucked away in a pannier, rucksack or even under the seat until needed.
For the over-trousers, try and choose a pair with velcro closures around the ankles, and preferably long ankle zips so that you can put them on without having to remove your boots. Don't tuck them in to the top of your boots because that way you're guaranteed to get wet feet.
With regard to the jacket, ideally choose something which has a front flap - preferably with press-stud closure - to cover the zip.
Non-padded jackets are fine to put on over your normal jacket; however they're normally made of very thin nylon so will offer very little additional warmth. Something with high-visibility panels would be advantageous as well due to the reduced visibility of rainy conditions.
Some riders prefer one-piece oversuits, although these can be more expensive than separates.
Stay away from rubber-impregnated oversuits. The waterproofing medium tends to crack and peel off after a period of time, and little leaks develop. They are also not that easy to fold into a small bundle!