I'd suggest choosing gear based on two things: your commute, and your knowledge/skills. There is no point in carrying something that you don't know how to use -- that includes tools, first aid kits, etc. If you don't know how to change a tube, why carry a spare? Also consider where you're commuting and how self-reliant you need to be. If you're doing a 20 mile commute on the prairie all on your lonesome, you probably want to carry the tools to fix most mechanical problems, an "ouch pouch" first aid kit to deal with minor problems, the know-how to use them, clothing appropriate for any possible changes in weather, and a cellphone to call for help. If you're like me and your commute is in a city, it's not nearly as necessary to be self-reliant in that way -- then it becomes a matter of choice whether you'd prefer to be able to (for instance) fix a flat when it happens, or simply lock up your bike, hop on public transit, and come back to deal with it later.