In the past I tried to buy the fastest lens that would fit my budget, favoring primes as one way to do so. As technology advances I am starting to wonder if I should still F-ocus so much on lens aperture. I am not suggesting that this is valid for all of us but for me, and possibly others who rarely crave shallow DOF, it may be worth considering.
In the past shallow DOF was only one of the benefits of fast lenses. A brighter viewfinder, more reliable autofocus and in general better optics and build quality were the main reasons why I aimed for faster lenses. However, new cameras now claim to autofocus at -3 EV (canon 7D mk2) or even -4 EV (samsung DX1, Sony A7S) of light and for mirrorless with electronic viewfinders the camera can brighten the image that is displayed.
Faster lenses tend to have better build quality, but my lenses are used 99% of the time inside a protective housing and I have yet to damage a lens. With regards to optics, I do not think that lens quality has ever let me toss an image, especially at my typical F5.6-F11. In fact, getting closer to the subject probably does more than a higher-budget lens could ever achieve. In addition, in the last decade many manufacturers have created a special line of semi-pro F4 lenses that give good build and optical quality.
Here is my stab at reasons to favor a fat lens over a fat wallet
If you like to use shallow DOF to isolate a subject
If you get everything else close to perfect, UW or on land, so lens quality gains importance
To capture more of your strobe's light output; increase battery life and decrease cycling time
If the faster lens has a shorter minimum focus distance
If you need a lot of light
ambient/red-filter video/photography under darker conditions or with fast shutter
to use a video-light instead of a strobe (but battling ambient may be the real problem)
to extend the reach of your strobes
... others ???
Maybe a better question is: how often do you find yourself shooting wider than F4 UW, and what other reasons/excuses to lust after fast lenses did I forget?