faster lens doesn't mean you have to use wider than F/2.8:)

for zoom lens, the widest lens so far is F/2.8(maybe F/2.0 soon). As we all know,better not use max aperture for best shooting result even for L-lens. so if F/1.4 lens down to F/2.8(there are 2 stops away), the lens will most likely maximize its best performance while F/2.8 lens will be much inferior at F/2.8.