While some might think the only importance of a transoms shape is to properly display the yachts name and hailing port, the truth is the transoms shape can decide your fate in a following sea.
Imagine running in breaking seas, crests so tall you can't see over them. When you are in the trough, you are surrounded by walls of water with no glimmer of the horizon. The wave behind you begins to approach your stern and is going to hit you, sending your yacht into a broach. The stern of the boat is pushed to the side by the wave as you lay beam to the sea, awaiting a breaker on the beam that will send you into a capsizing roll!
The larger the area presented to the wave, the more force the wave will be able to exert onto your vessels transom. A large flat transom will present a large surface to be pushed by the wave. The polar opposite exists in the form of a Double-Ender. These boats have a stern that comes to a point just like the bow comes to a point. If a wave were to hit their transom, the stern would cut through the wave the same way the bow cuts through every wave it encounters while sailing.
The modern trend of Sugar Scoop transoms is very convenient, people can easily climb aboard or jump into the water from the cockpit. The back of the cockpit is missing and everything simply flows right into the water. When a large wave hits from the stern, this wave will just as easily board the boat through the large open stern scoop, swamping the aft part of the sailboat. Boarding waves can become a very wet ordeal in a hurry.
The other option is the long overhanging stern common on older CCA boats. This will help pick the stern up as the wave approaches while the small area on the transom (which serves to hold the name) will offer little push from the wave as it gets hit.
Which stern would you rather have as you get hit by a following sea?
Large Flat Stern
Sugar Scoop Stern
Double ender Stern