I used a tractor for some of my birds over the summer, 2x2x6 with the two feet on one end a house area with wire floor and the rest on grass. With four birds in it, I had to move it every two days. Any more than that and I had to move it every day. That meant shooing them all into the house area and shutting them in while I moved it to avoid damaging their legs. Not a huge deal and they liked being on grass.
I live in the Pacific NW, so now that summer is over it is not an option and everyone is on wire. It is cleaner for the birds, although I'm sure they'd rather be on the ground. They all have places to get off the wire, such as sandboxes and solid floor hutch areas. To give them a dry pen on ground (which wouldn't be a tractor) would mean a big project and more space than I have available. I do applaud people who manage to keep them on the ground, though.
The idea of moving a pen on wheels is okay, but for me it's better to have them stationary and clean underneath every few months and toss it into the compost bin. For pens with trays, I clean them every week, which is just a matter of scraping them out into the compost and not a major issue especially if they've scattered sand all around the trays.