The mast would hang off too far off the back of the boat if you left the mast on the mast step. The front of my trailer has a post with a v-shaped brace at the top for the mast. Even with this, it hangs several feet off the back.
Most sailboat trailers have a mast crutch at the bow of the boat. This is where the forward end of the mast lies during trailering. The vertical bar on the trailer that makes up the mast crutch is also where the bow winch is mounted.
Unfortunately, my trailer does not have a mast crutch. The previous owner built a mast crutch that sits in the mast step during trailering. Basically, it's a square piece of lumber approximately 24" long with a large notch in it to cradle the mast. It works well but it's a pain to rig since it requires lines tied from it ot the forward, aft, starboard, and port to assure that it stays vertical. I imagine that I'll eventually replace it with a standard trailer mast crutch.
The same previous owner also made an aft mast support, which is not as intricate or pretty as the PVC support, however it is equally functional. My aft support is a 2" x 6" piece of wood with a large notch cut in it to cradle the mast. The wood cradle mounts to the transom motor mount with two bolts.
See the attached photo. Sorry about the clutter. The boat is currently stored in my garage for the winter. I just poped the cover off the boat and attached the crutch for a quick photo. Photos of the aft crutch will follow.