Dan,
I realize it is not neccessary, BUT, (BIG BUT), for all the trouble it is to carry a hunk of chain,jack up the trailer axle,take off the flat tire,chain up the axle,bolt the chain together so the axle is up out of the way,let down the axle and go on your way.......
or ..
jack up the trailer,pop on your spare and go on your way....
Common sense and forethought is to have a spare.
Why the salesman would tell somebody they don't need a spare is beyond me(not ME!).