You are spot on. We have several bicycles in my extended family and I have the tools to repair all of it, but when we bought or built the bikes we looked for the widest compatibility possible of the spare parts: 2 BB types, 2 chain types, 2 fork and bearing types and 4 wheel sizes for ~ 10 bikes. The only "rare" tools are a Shimano Hollowtech 2 wrench and the "square BB" one, the rest are common tools most people have in the house.
