Agreed. With a normal or tele zoom lens, f/2.8 is useful both for the additional light (compared to other zooms), and for the additional OOF blur for subject isolation. With a UWA zoom, f/2.8 is really only useful for the additional light. You're right about the museum shot, and if I'd had my tripod along for the walk, I'd have used it (and an ISO lower than 3200!). But also for indoor shots with people, the extra stop compared to f/4 helps for subject motion, a problem that a tripod and a long exposure would not solve. One other minor point is that all lenses benefit in IQ to some degree by stopping down, and the 'sweet spot' isn't usually a fixed aperture, but rather a given number of stops down from wide open. So, a wider starting point means more light and possibly less diffraction effect when you hit the sweet spot. For example, the 16-35/2.8 II hits its sweet spot at f/4-f/5.6, while the 17-40/4 hits its sweet spot (such as it is) at f/5.6-f/8 (and f/8 on 18 MP APS-C is pushing into diffraction territory.