Also depends on the type of tire used. If the tires are 5-6+ years old and the usual load C/D/E ST maypops then they don't make a good reliable spare at that age.
If the tires area LT or a commercial grade load G or H ST then they make good spares for several more years....assuming their protected from the sun.
Now if the trailer sees maybe 1500 miles a year then cheap quality low cost tires may work best.
On the rv trailers I've owned and several GN or bumper pull service trailers I never buy a new spare.
Run a quality tire on a road trailer and the spare may never roll on the ground.