OK, at the risk of being stupid, will someone put me out of my ignorance and explain what's going on here!
On the rims I converted from tubed to tubeless for friends, they have been able to run tubeless with no problem after with a couple of exceptions.
I made sure they also carried a
spare tube or two (and a tire boot) in case of:
a) a puncture too large for the sealant to heal (or in case the sealant had dried and was no longer liquid, a problem easily fixed with periodic refreshes)...
...or...
b) in case their inflated tire had gone down and they were unable to reseat it in the field with the pump they carried. I've had good luck seating tubeless tires with ordinary floor pumps but it can sometimes be a different story in the field with, say, a mini-pump.
Depending on the state of the sealant, a field swap to tubes can be messy or inconvenient.
While running with the tubes in place, the usual cautions about proper inflation apply as running inadequate pressure can result in a pinch-flat.
For either of these occasions, a spare tube can make for a quick and relatively easy bailout option as a temporary measure. You do of course have to remove the tubeless valve in order to make the switch.
Some people in areas with goathead thorns run ordinary tubes filled with sealant. Of course, this is not a tubeless system, but a self-sealing solution. These tubes would need to have removable valves to allow filling with sealant.
Best,
Dan.