And by the time any helpful info that the buyer may have discovered on the UVIP (a brand, problematic ownership history, or a lien), it’s a bit late to do anything about it.
You nailed it on the head.
That is the single most important reason to get one, and if the seller refuses you need to wonder why.
It's 20 bucks. If you really want the bike get the vin before going to see it and buy it yourself. It could save a lot of headache down the line.