Originally posted by ROW Live in South Australia. Climate here can be hot in summer and I go bush a bit, where it can be dusty.
I grew up in Adelaide, so I can relate. Last time I was there, it was pushing into the mid forties in daytime. WR for all but the * lenses is mostly geared towards rain rather than dust AFAIK, but it can't hurt to have a "sealed" package whatever the circumstances, and pervasive stuff like dust is a good reason to want a single-lens solution. I also lived in Melbourne for a while. Had I stayed there or moved to a larger city in Canada (where I live now), I'd potentially have gone with the 16-85 as the wide end would have been handier in the CBD (in which I spent a lot of time). As it is, where I am now is flatter and more wide open, more of the shots I take are at the long end, and for you I suspect things are the same; IMO the 18-135 suits those conditions far better as a WR one-shot deal.
Not to mention that during the daytime at least, there is usually more than enough light to keep a camera with an 18-135 zoom lens happy even at f/5.6. If there isn't, the rain (or dust) is probably about to get so bad that even a WR setup is in peril.
Back when I had an *istDL, weather sealing wasn't a thing on the camera, was therefore irrelevant on the lens, and the 18-250 thrived (throve?); as I just said, lack of light was hardly a problem, and even f/6.3 could see the shutter speeds up to reasonable hand-holding levels on that camera.