You never know when your tires ditch you. If you get stuck with a flat tire in the middle of nowhere, your spare tire is going to be pretty useless without a lug wrench. Yes, changing the tire is not like watching your favourite movie but it is far more better then stucking in the middle of a road. So, always carry a lug wrench.
Thanks! Your local auto parts store sometimes might carry extendable lug wrenches (1 foot to 2 foot lengths) with a 3/4 socket in 17mm, 19mm, 21mm, and one more I forgot. You'll only need the 17mm for your w123. That's what I have, and save my tool kit as part of the "original items" bin (among the first aid kit, owner's manuals, etc).