I carry two sizes of allen keys besides the leatherman. They weigh practically nothing. It certainly is overkill but from an effort point of view, I didn't have to buy another multi-tool. Plus the pliers.... Are there bike multi-tools with pliers?

edit: I'm not advocating that people buy non-bike tools, but just saying it is not a bad option if you already own one or something. I've found no need to get a bike-specific tool. My spare kit contains the leatherman, some plastic park tire levers, and 4 & 5 mm allen wrenches (besides tube patching stuff and tubes).