Went ahead and ordered some this morning. Kinda sucks because I've got 4 trailers with 14 tires total! But I think the covers could stretch out tire replacement intervals due to the sidewalls weather checking and possibly pay for themselves in the end.
To protect the tires, I normally take pieces of scrap plywood and cut them to fit over (or under) the wheel wells. You just need enough to keep the sun and weather off the tires.
The problem isn't tires and dirt it's tires and sun. One of the worst things we can do with our pieces of equipment that don't get regular use is to leave them out in the elements and yes sun will raise cain with tires.