Everything trumpet power said about zooming and the aperture adjustment of zooming is true. The thing is though, and you'll see this fairly quick, is that zooming with a lens on a dslr looks like S___, due to the cmos sensor. Rarely does a zoom on a photography lens look good, at all. This is one of the main reasons people keep saying to get a prime and use your manual zoom (feet).
Primes are great. However fast aperture zooms are more workable in terms of overall range.
I have 3 f2.8 zooms covering 11-200mm on APS-C. That would take a lot of primes. It also means I spend less time changing lenses and more time shooting (I work with a producer, thats the crew, pretty much run and gun, trying to shoot an interview with varying frame sizes would be very difficult without zooms)
Whilst I don't want to live zoom, I often want to change shot size without changing lenses.
So don't write of zooms entirely. But if you are going to shoot with a zoom lens, make sure it's constant max aperture (on a 18-55 lit lens for example your shot is 3 times darker at f5.6 than at f3.5... ) and if you can afford it make sure that that max aperture is f2.8.
There are times when you won't want to add any more ISO. And as your shutter is best locked at 1/50th or 1/60th for regular recording... the extra aperture is invaluable.