I care about about getting the shot, that may mean having enough shutter speed, or having the ability to quick change focal length. The differences between a high quality zoom and a prime lens are not that drastic.
Primes are small and light and also are typically faster than nearly any zooms.
My zoom is a 70-200 2.8 IS and I also have primes in that range of 85 1.2 and 135 f2. The 85mm is more than 2 full stops faster than my zoom lens at that focal length and the shallow dof it produces as well as the low light I can shoot in cant be matched by a zoom lenses. When its the right tool for the job, its the only tool for the job, if you know what i mean.
I will say though that some primes do offer a level of sharpness that zoom lenses just cant match. my 135 f2 and 85 1.2 as well as 300 2.8 IS are very useful for their speed and bokeh but you'll never hear my complain about the image quality of them. Its as good as it gets and when the choice is there to use the prime or the zoom, and i have the time to set up a shot, the primes are always the clear choice.
Even at the same focal lengths, the primes typically have a bit better color, smoother bokeh and more sharpness.
The real answer is to have a mix of both, theres no rule saying you need to be just one or the other.
sometimes I need a fast versatile zoom lens for an unpredictable day of shooting and thusly I bring out my big 24-70 and 70-200 IS zooms. For other events I know that I will only need a certain focal length for a portrait shoot so I take my 85 1.2 and 135 f2. For other events I may need to travel light so I take a wide angle zoom lens and then a longer prime in case I need the reach for a rare shot.
Theres a use for every lenses made and Id love to own them all. Remember though that even if you do own every EF mount lenses made, it wouldnt really make sense to take them all with you so you will always have to pick and choose.
having an assortment of zooms and primes just gives you that much better chance of custom picking the "right" kit for a given day
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