f/2.8 is not that thin DOF as you go to UWA. A 50 mm FL at 6 feet away f/2.8 gives DOF 0.78 ft (very thin, and difficult to manage), whereas a 24mm FL gives 3.4 feet, which is more than enough. At 15 feet away for a group, 24mm lens gives a whopping 36 feet of DOF. The razor thin concern doesn't apply at ultra wide.
An extreme example here: 24mm f/1.4 at 15 feet still has a DOF of 11 feet (but Canon's 24 1.4L is very soft in the corners at 1.4, different issue).
16mm f/2.8 at 6 feet away still has a very easy to manage DOF of 11 feet. Even as close as 3 feet, gives about 2 feet DOF.
So, f/2.8 really can help indoor photography for ultrawides without causing DOF problems.
yea. so f/2.8 can help isolate the subject from the background as well. in your example, the DOF extends from about 2 feet in front to 9 feet behind, which may be pushing it for subject isolation but still doable (you would more likely be stepping back and zooming in to 21 mm for example, for better results -- But to continue the example: at f/4 (still 16mm and subject distance of 6 feet) you loose almost all hope of subject isolation from the background because everything 34 feet behind the subject is in focus. so in this particular example, the f/2.8 lens has a hope of capturing a venue feature like a candelabra or whatever, with some isolation from the background, but the f/4 lens has little hope. I doubt very many people/group shots are taken at 16mm and 6 foot distance... but I'm not a wedding 'tog so I'm open to correction here
The things with weddings is that your often forced to shoot in less than ideal situations. I use the 24mm alot for family group shots --- why? it's a trade off...you want head to toe, but they want them in the church in front of the alter. If you slap the 50mm on, yes, less distortion but you have to step back so far that you get the pews in the shot, so your stuck doing a hip and up shot, or, go with the 24. Or if it's really tight and the bride and groom don't listen to me when i say it's better to do the shot outside (oh, it's too hot, or grandma is in the wheelchair, or any number of on the spot excuses that can happen) - yeah, go with the 16-35...it isn't ideal but that's wedding photography...