Last year I shot a wedding with only the 40mm pancake. Admittedly, I wasn't the hired photographer and I definitely missed a few shots, but I didn't find the experience as limiting as expected. The key to using primes in a fast-paced event setting is to have multiple camera bodies, but most people seem to recommend 2+ DSLRs for weddings anyway, regardless of lens choice. Of course a zoom is going to offer more flexibility in terms of focal length - to what degree this is desirable is debatable. In theory, the photographer will be able to instinctively "see" what combination of focal length and camera position is ideal, but with a zoom I often find myself neglecting the camera position and simply zooming to obtain the "ideal" framing, often at the expense of ideal perspective.
Cost is another important issue. A 2-zoom setup with f/2.8 aperture would run me over $2000 on a crop camera. Or, for $1000, I could cover most of the bases with a 28mm f/1.8, 50mm f/1.4 and 85mm f/1.8. Another possible option is "prime wide, zoom long." The wider focal lengths are usually easier to "foot-zoom" than long lenses, and thus one may want to use a selection of wide-normal primes along with a telezoom. Of course, all depends on the situation and skills of the photographer.