basically having a wide aperture available is like a condom
its better to have it and not need it than to need it and not have it
f2.8 gives much nicer shallow depth of field shots than f4 but probably both eyes will be in focus the one eye in focus thing is more the domain of fast primes
typically at weddings I'll either be shooting my 85 at f2 maybe f2.8 for more people in the shot and occassionally f4 or 5.6 but really i use this lens for portraits and couple shots
I use this lens more now than i do my 70-200 f2.8 since at f2 its sharper than even the 70-200 is at f2.8
I'll shoot the 70-200 at all sorts of apertures depending on the shot.
for the bride and groom walking down the isle its much better to stop down and play it safe
since using servo and how fast it all goes off there isn't alot of time to mess around and a redo isnt an option
the 16-35 i'll shoot at everything from f2.8 to f11 depending on the shot having 2.8 on this lens is a godsend with the 5Dmk3 for receptions (same goes for walking down the isle shots with this lens, stopped down is better) and its a fantastically fast focuser in low light, really really impressive on the 5Dmk3.