So here's the thing: if the 18-135 is not so good from 70mm onwards, and you do want good image quality, the 17-70 offers that in the same range, plus 1mm more at the wide end and a constant aperture. Also I've read that the 18-135 is f/5.6 from 78mm on.
Regarding the importance of edge sharpness (I won't go as far as extreme corner sharpness), I have also put the 17-70 at f/5.6 or f/8 and 70mm and got tack sharp landscape shots of an island in Khao Sok national park. So to me it is important, at least when you close the aperture down a bit.
As for range, I also brought the 55-300 (and the 35/2.4 as well). But, despite getting a few nice shots with the 55-300, I more often wished I had something wider.
Yup, continuing to make a case for this lens.