My family has been feeding round bales as long as i can remember. We have been doing our own hay for the last 10 years too. The thing about round bales, IMO, is that it has to be cured correctly before being baled, otherwise the whole bale will mold, from inside out, it even may look like a nice bale on the outside, but if you pull a handfull from the middle, that tells the tale. We will not feed these bales to our horses.
Now if a bale is cured correctly, but then sits in the rain, it will slowly rot, outside in, the part touching the ground would be the worst. After the first couple layers, the hay is still good, you would check for this the same way, by pulling out some from the center. Horses will not eat the outside, they will go directly for the centers, and they will eat all the good stuff, leaving the bad. This we have fed to our horses, BUT we put a large number out so they can pick and choose. We can tell when they would need a new one because they stand by the gate closest to the house. None of our horses have ever coliced because of the hay.
I can see where y'all are coming from, about why you wouldnt feed older bales because it does have leave some waste, and its not really worth the money/effort. From a producers standpoint is : we have excess bales, no one has picked it up... so leave it out in the field to rot, or bring it home so our horses can eat whats left of it... but when we bring it home, we place them off the ground, and under tarps.
Plus roundbaling really only takes one person to produce...square baling takes 2-3, and its more labor intinsive. For awhile we didnt have a tractor at home to lift the bales, so we would take the ATV and hook a chain around the bale and drag it, or just roll it out by hand...