Sometimes - it really depends on what you're shooting - for how sharp you really want/need the picture to be.
Having a fast (or long) lens can add in a lot of bokeh which can make the in-focus object 's sharpness stand out.
For some wide / far landscape - you might not need to have a tack-sharp image - as the DoF is going to vary anyways - so not everything is going to be tack sharp.
If you're doing macro/wildlife or a specific subject - then you might want that detail.
These were taken - handheld with a manual focus Rokinon 85mm f/1.4 @ f/1.4. The second one isn't as sharp as it could be - slight movement caused OOF
If I had stopped these down a bit (especially the outdoor shot) - they'd be much better.
In this case - focusing on an object - typically you'd want a VERy sharp lens. I'm only an amateur, so I don't mind criticism.