The theory goes that a prime, i.e. non-zoom lens is a lens which has been designed for the purpose of it's focal length and is superior for that purpose. For example an 85mm lens is designed as a portrait lens and since it was made for that, is well suited to its purpose.
Since the late 80's (?), zoom lenses became popular among photographers for the versatility that they afforded. However the lenses were a compromise in that it is very hard to squeeze the best characteristics of each focal length into one lens. All sorts of technical issues arose and had to be dealt with.
Since the 90's, zoom lenses have come of age and are optically superb, however it is far harder to build a zoom lens than a prime and most still consider zooms a compromise optically compared to the equivelent prime lenses. Maybe not enough of a compromise to deter but still not quite as good in the majority of cases.
The more magnification a zoom has, the more technical issues have to be addressed. A monster lens such as the 28-300L lens is an example of such a super zoom, costs over $1500 and is probably still an optical compromise compared to smaller zooms, i.e. it will be less sharp in a side by side comparison.
Most pro photographers are even wary of lenses such as the 17-85 and 24-105L in that the zoom range is so high relatively.
I doubt that MR would chose the 17-85 IS over the 24-70L if optimum sharpness was needed. A 17-200mm zoom, especially as wide angle zooms are the very hardest to design, is a bunch of compromises on top of compromises. If it were to be a top of the range lens costing thousands of dollars and weighing in at 3kg then maybe. At the prices these lenses sell for I wouldn't touch one with a long bargepole! :cool: