we do it because those trucks get used at the farm more than anything...just the trucks keep the plates on all the time, and the chipper, and slasher, and a couple other trailers share the same plate, as they only move from site to site, it would be insanely expensive to register every trailer, and farm truck with commercial plates.
we're not a huge company with 100's of trucks, (its only 3 - 4 guys) we have a few trucks and we keep them in top shape because they need to work when we need them, wouldn't be the least bit concerned with dot having a problem with our trucks!
I see what your saying though...theres a lot of guys out there that drive death traps down the road and don't think anything of it...