I have found that carrying a spare jack is a wise and valuable thing. I always keep a spare, cheap scissors jack and a few 2 x 8's handy with the trailer.
It's not about fixing flats, it's about lifting weight when there's no other way. Try lifting that tongue when it's on the ground and the built in jack is bent or busted. A couple months ago I needed it to help stranded motorists I came upon who were stuck in a desolate area. Anyone notice that often there's no cell phone service where we go camping?
"A billion here, a billion there...add it all up and before you know it you're talking real money." Everett Dirkson