You only need a 12'6 for technical (BOP) races
Most races are more suited to 14 foot class. Its faster so gotta be more fun.
If there is a 12'6 class it is to cater for both lengths or geared towards the Pro's.

Saying that its easier to store a 12'6 and more fun on the waves.
So horses for courses...

Many people who buy a 12-6 go on later to get a 14.

Few people who have a 14 go on to buy a 12-6 (unless it is specifically to do BOP-style races).

Draw your own conclusions about which length is generally more rewarding to paddle, whether you are racing or touring.

In Europe between the 8.5-10km with really long ones at 20km.
At our national races you see that the top of the pack settles most of the time within the first 3-4km. Behind them in the big group the there is a lot more position changings, but they also stabilize around the 8km.Only difference are the 20km+ races where you see the guys that are really fast in 10k sometimes come short when the race become longer and slower starters have more time to come back on on the faster ones.The ultimate Long distance flatwater race is the 11 city tour in Holland. 5 days of +/-40km paddling or "just" 220km non-stop!