Nesting tools are great. So one half is Allen, the other fancy stuff like chain tools etc. Often the multi tool get in the way of using them with an Allen bolt. I often carry free single Allen keys for the 4mm seat post and brakes.
The Shimano Pro tool is nice and light.
The other "problem" I have is where to put it. Normal is in a bag under seat, and that bag is strapped on and full of other stuff that is a pain to re-pack. I'll often have just an Allen key or two in my jersey.
A tool that snapped onto seat rails or chain stays would be quick and easy to get to. Maybe a Gamin mount snap, you could make it part of the lights - now that would be cool.