At first I wanted to have anything that I might conceivably need, right there on the bike at all times. Any more, after thousands of commutes, I don't carry much in the commute bag except for daily wear and a few toiletries.

To be honest, strictly on commutes all I really need is a tube and air. But since the bike is used for longer rides also it needs tools, so they stay in a saddle bag, one for each bike. Just tube, air, patches and multitool.