I have a BFG that was damaged at the bead upon removal from the previous set
of wheels. It is still a great tire with tons of tread. It just leaks air
now. I'm thinking throw a tube in it, and since it will be used as a spare
anyway..... it should hold air well then and serve it's purpose right?

Are there any flaws in this statement? Are there problems with tubes?
Is there a reason why 4 tire shops told me today that they can't legally put
a tube in a tubeless tire, but all suggest that I do that?

-Craig