On my hybrid I have a decent sized bag and carry:
lock
lock keys
house keys
wallet with ID, donor card, debit card and cash
spare gloves / mitts
multi tool
puncture repair kit
2 spare tubes
pump
spare waterproof top
spare tee shirt
sometimes a bit of shopping on the way home
sometimes my camera if I want to take better photos than m phone can take
2 cereal bars (in my pockets)
lens cloth for my specs (in my pockets)
tissues (in my pockets)
water bottle in cage
On the road bike, I only have one of those small compact pouch type bags that go under the seat which can just about take a tube and my wallet, but by using an extra water bottle, I manage to get the multi tool, pump, one of the 2 tubes and puncture repair kit into that, but I don't have room anywhere for the
spare tee shirt
waterproof top
camera
spare gloves
lock
shopping
On 2 occasions I have been unable to fix a puncture because my pump broke, once I walked the 4 miles home and the other I was able to get someone to pick me up, but it does worry me that if I was out on the road bike and couldn't repair a fault, I don't have a lock so I would be unable to lock my bike, get a taxi and come back.
However, on most of my rides I don't stray more than about 30-45 minutes walking distance from a train station from where I could take a train home (albeit from some places it would be a circuitous train journey), so it might not be the end of the world, but the thought of walking that long on cleats doesn't appeal to me.