My Crossovers are the ultimate for majorly muddy conditions during Bay Area winters (feet sinking in several inches into deep, goopy mud) and for quick runs when I'm up snowboarding in Tahoe.
The built-in gaiters keep out the mud or snow.
Being a trail runner, getting wet or dirty feet is not that huge a deal for me, but I'm an ultrarunner, so often out there for hours. Wet feet sort of get old after 2-3 hours, and really gnarly after 4-5. It's also nice to be able to avoid wringing out the muddy water out of my socks 20 times before I throw them in my laundry pile.
I'm not sure if I agree with the 1.5 sizes up recommendation. These fit like the Crosslites, and not the tight fit of the Quantums, Electrons or Verticals.
(For full disclosure-- I am a La Sportiva Mountain Running Team member)