I have gone through quite a few lens reviews and questions on this site that recommend expensive lenses over their cheaper counterparts. From what I've seen in the reviews, there is a marked improvement in sharpness (esp. at 100%) for the premium lenses over their cheaper counterparts (E.g. Canon EF-S 17-55mm vs 18-55mm). Besides this, many of the premium lenses also have a wider max aperture which may be fixed throughout its focal range, and this is a tangible benefit to composition. Build quality is another benefit, but that doesn't impact image quality.
So, my question is what other benefit does a premium lens provide over the regular lenses? Do premium lenses have any impact on image composition & photographing technique (I'd suspect that the added bulk can be a hindrance at times)?
Note that I am not considering specialized lenses like 600mm and the likes which do not have any regular counterparts. Also, by counterparts, I mean lenses having similar focal length range and apertures.