Among the reasons to use f/1.4 lenses over 2.8 zooms or primes:
1. Need for shallow depth of field
2. Balance ambient with flash, particularly in dark environments. The f/1.4 lens will allow for lower ISO and/or lower flash power to balance the ambient with flash.
3. Similar to #2, desire/need to keep ISO lower, particularly in low-light situations.
4. The need to manual focus. An f/1.4 lens will be brighter and allow a clearer distinction for focus plane in most instances. There are challenges to this in many newer cameras due to standard focus screen designs, but many can be replaced with brighter or split circle designs for manual focus. Manual focus with f/2.8 lenses is an absolute chore on average DSLRs unless you have the time to use live view.
Images are always a trade-off with respect to technical capabilites of your equipment based on the situation and type of image desired. If you shoot often in situations which call for the above needs, you simply can't do with 2.8 lenses.
On the other hand, many, if not most landscape or general shooting situations call for lower than 2.8 aperture and therefore the reasons for using a faster lens are significantly diminished. Still, you may decide you just like the way a particular lens performs. Maybe you just really like the way the Zeiss 35 1.4 draws better than, say the 24-70 f/2.8 L II zoom. (You probably wouldn't really know this unless you own or have extended access to a particular lens).
In short, sharpness is just one of many factors in choosing equipment to accomplish your photographs. Only you can really answer the question of whether you need it or not, based on the types of images you are trying to make.