Good morning, Stephen;
As David Goldfarb and others have mentioned, there really are some differences between the 24mm and 28mm lenses. I also feel that the 28mm is my limit with just taking a lens out, putting it onto the camera, and shooting. I do prefer the 24mm lens for interior shots of rooms. And, the 24mm lens is were I start to feel that I am using a wide angle lens, and I need to start watching the horizon and other lines for signs of curvature. If I drop down to a 21mm or a 20mm lens, then I know that this is a wide angle lens, and I really need to watch which way the lines and things are going. To me, the 21mm and shorter lenses really are special purpose lenses; even my 17mm "Rectilinear" lens must be handled with great care. The 16mm is where "special effects" hold sway.
For me, the 28mm lenses are very good lenses for all kinds of "normal" things; landscapes, general scenes, et cetera. The 24mm lens is my preference for interiors, and it comes in handy for wide landscapes with careful use. What this probably comes down to is; I think you need both.