No problem using tubes in tubeless tires, you may have to if it is a spoked rim, or other non tubeless rim that won't seal without a tube.
However, if you do not have to use a tube, you shouldn't for the following reasons,
Tubes add unsprung weight (ok so it ain't much)
The friction of the tube within the tire creates more heat, causing tires to run hotter (again this is slight under normal conditions)
The number one reason to not use tubes is that tubeless tires without tubes rarely blow out or lose air rapidly. The usual scenario is they get punctured by a nail or thorn and you get a slow leak that will hold air for days or weeks. When a tube is punctured it loses air pretty much instantly. A tubeless tire is way less trouble than a tubed tire.
Keeping in mind that it is OK to use tubes when they are not needed, it is just better not to.
Using a tube for a damaged tubeless tire is a very bad idea, but I suppose if it gets you out of a jam it is OK temporarily.