Here's my problem with running UST without sealant:

Say you run over a thorny bush, and you get multiple thorns/punctures in your tire. Your tire goes flat. You remove the valve, throw in a tube. And...it goes flat, too. Why? Because you didn't know about the multiple holes. Just one.

With sealant, it takes a heck of a lot more to actually go flat. If there are multiple holes, you'll know when you see the sealant spraying or bubbling out. After that, you can wipe out the sealant, patch the tire, re-seat the tire and inflate.