Well, Ian doesn't necessarily use them - although his assistant does!
While Ian is getting the shots, his assistant (or me, for this particular Apprentice) is running around with reflectors, sun-swatters and step ladders.
I've seen a fair few wedding photographers use similar kit and quite a few with loupes around their necks; particularly ones who record video footage as well as stills. To be fair though, that's the first time I've ever seen a wedding photographer armed with a mono viewer! I thought those things only existed in museums now!