Avant etc are OK in wet, wider tyres the better. You can't expect them to keep running in the shittest conditions though, just as you can't expect a wheeled dumper to keep running.
You'd need some seriously specialist work to warrant those tracks. So much to break on them, and I bet they give a hard ride. There was one on eBay not long back with not many hours, so I reckon they're nowhere near as cool to own as people think they look!
If the ground is that bad that you need tracks, do you really want to be carrying pallets across it? Use a tracked dumper instead. Same goes for moving muck. A digger with auger/postknocker/whatever will be much more efficient in the slop as you can position the tool with the arm.
So what good actually is a tracked Avant?