OK, thanks everyone. So it sounds like it is a safety issue, which I know about, vs can damage trans or clutch except one reply said is bad for throwout bearing if clutch is depressed.
I skimmed the subsequent posts, but don't think this has been addressed...
The safety issue is fairly apparent, but there IS a damage concern...
When you depress the clutch and coast, you are still spinning the clutch disk and transmission, just as if you were revving the engine higher to achieve the same speed.
So look quickly at some numbers: Let's say you were in a gear that would get you 5mph at 2500rpm. You step on the clutch and coast down a hill, hitting 10mph. At 10mph, the clutch disk and trans main shaft are now spinning 5000rpm. Hit 15mph, that's 7500rpm on the clutch disk and upper bearings. That's too fast for both, most likely.
The real worry is that you're in a very low gear, let's say one that lets the tractor do 2mph at 2500rpm. If you clutch and coast to just 10mph, and the clutch disk and input shaft of the trans are doing 12,500 rpm's. That's about where an automotive clutch starts thinking about flying apart. I would bet a tractor clutch won't go that fast without tossing the lining...
So there's your damage concern. Don't coast faster than the tractor can go on it's own as a general rule, but if you somehow feel that you must, take the transmission out of gear so you don't destroy the clutch and upper bearings.