Even with proper maintenance things break, either on their own or with help. ;-) I view carrying tools and spare parts like insurance. I would rather have and not need them, than need them and not have them. If you ride close to your truck you probably don't need to carry much. I personally hate riding a bike or with bikes that could perform properly with a few tweaks. Weather, temperature and altitude and altitude changes are a fact of life and you cant always predict where you will ride. We ride with 2 adults and 4 young adults and we typically need to do some tweaks or repairs on the trail.
Wearing a "real" backpack that fits right and properly distributes and supports the load on the rider is critical. I have a Dueter and love it. The below list can be changed depending on the ride.
large adjustable with the handle cut short
small adjustable wrench
small vice grips
Swiss army knife
small needle nose plyers
shorty metric wrenches 8, 10, 11, 12, 13, 14 and 17mm
allen wrenches
multi screwdriver with quick change bits.
Jets mains and pilots, spare needle, clip and jet tool
small hose clamp,
3 ft of 1/4" fuel line
spare master link
tube patches
small tire pump
misc nuts, bolts and washers
zip ties
electrical tape
few feet of wire, quick connects / butt connectors
small first aid kit
large garbage bags- work great in a pinch for rain gear and to stay warm
paper towels in a zip lock bag- works to clean everything from one end to the other ;-)
spare riding gloves
LED flashlight
power bars
crazy glue
epoxy putty
lighter
space blanket
2 spare spark plugs and plug wrench
registration and insurance card
tow rope
Without water it weighs in at 10.9lbs