Although I mostly shoot a 50mm Summilux on my Leica M2, I do enjoy a different focal length every now and then. What I don’t like is adding lots of weight to my bag. The one lens that tends to surprise people with a proper (heavy!) DSLR-kit the most is my Tele-Elmarit 90mm. It’s tiny and light, and you can keep it inside the pocket of your ripped jeans if you like.