Six years after my comment in this thread, and it surfaces again. I've come to use 28mm as a general purpose focal length, due to the Ricoh GRD III, GR and GXR 28, as well as Sigma DP1, and 28mm on a rangefinder.
But I've spend a lot of time with 24-21mm, thanks to cameras like the Panasonic LX7 and LX10. 28 is fine for day to day, but I prefer 24, 25 and 21 especially for travel.
For me, I like to include a lot of context in my travel images as they are visual reminders of everything that occurred, and 21mm is great for that. 28 can be quite intimate if you're relatively close to the action.
When it was working properly, I loved using the Zeiss 25 Biogon on the M9 as a general lens. More context-grabbing than 28 but not as hard to manage as 21.