Another good reason if AF. Top tier bodies have several focus points, some sensitive to f/2.8, some to f/4, and some to f/5.6. Non-top tier bodies typically have an f/2.8 center point, and f/5.6 everywhere else. Buying F/4 lenses means (I think, but I could be off), that unless you use the center AF point, your AF sensors aren't sensitive enough for your lens... the camera might think it's in focus, but it's not.

Also, typically, the non-f/2.8 lenses (aside from the super teles) just weren't built to the same standard. Canon is putting out a series of high quality F/4 lenses these days, but it wasn't always the case. The 24-70mm f/4L IS is a weird case. Yes you get IS and the dubious macro mode, but it's not nearly as sharp as all the other lenses Canon (and Sigma!) have released recently, which makes it hard to justify the non-trivial price. Especially since you could get the 24-105mm f/4L IS for less, which also wasn't the sharpest (atleast my copy wasn't!) but had more range. Most people who are willing to invest in a lens aren't willing to invest in a just okay lens. They'll save their pennies for a great one.