Some of us want a lens we can know will perform in the rain and snow. Is this as weather resistant as Canon's L-series lenses? Can this lens go to the windy beach and not get clogged with sand? I've had Canon's 24-105mm f/4L IS in all sorts of conditions, and I have not thought twice about it. Dumb luck or specs, I do not know.
Is there a reliable rating system for such a question?
Hey, no weather sealing, not to mention the double-barrel zoom makes an extra area "exposed" for potential element influx. I don't think Sigma has an "All weather" rating, and neither does Canon (not even all the white L lenses are weather sealed).
"Moreover, with dust- and moisture-resistant construction"
Perhaps not the sealing for a downpour or sandblast, but still tight and "resistant"