The keychain mod is a good one, most of the drivers work fine in this capacity. I tried it out for a little while, carrying a philips and a large flathead but I never used them so I retired them to the collection box for the time being.
I'm basically saving my spare parts until I get struck with a genius idea or get better tools to make some modded carriages.
I'd imagine you could always sell or trade the spare parts. I'm secretly hoping someone will start a spare parts emporium, wherein I could buy all the extra parts I needed to really customize a wide range of tools.