Every Lens has their strong points and weak points. The Primes are just cream machines ... in terms of Bokeh, you gutter-boys.
The 17-55 is a great lens that it can take such great wide shots. And with cars, that's usually a good thing to get the whole car.
I shoot with my 17-55 2.8 alot at car meets to get the whole ambience of the meet and to get whole cars, then as I walk back I switch to one of my primes to capture the details I missed.
The 17-55 is pretty good for family and large group shots and the primes like the 50mm is good for the details, like mom's earrings or a wife's wedding band.
My 17-55 Example(s):
My 35 1.8 Prime Example(s):