I was speaking to Andy Longshaw at the SkillsMatter Agile and Lean eXChange conference. He told me a great story. There are two guys at a train station in India. It's hot and they're having a cold beer. The train pulls into the station. In no time at all its absolutely jam packed with people. Despite being "full" people continue to get on. People hang onto the side. People hang onto the front! People climb onto the roof! The guys decide to have one more cold beer. They miss the train. They ask a guard "when is the next train" and the guard replies "one month next Wednesday!"
There is a wonderful underlying dynamic to this story. It's because there is only one train a month that it's so overcrowded. If the train ran more frequently it wouldn't be so full because passengers would know it ran more frequently. Perhaps in the past there were two trains and they ran twice as often. But maybe one broke down and for whatever reason never got fixed. So now there is just one a month. And every time it runs it's dangerously overloaded. Because it's overloaded it's more likely to break down...
If you release every 12 months and a feature doesn't make it into the next release you'll have to wait at least another whole year for it. So there's pressure to add loads of features into the next release. And everyone knows that 12 months is optimistic anyway. So there's even more pressure to add even more features into the next release. The release train gets full. And the more overloaded it gets the more likely it is to slip. And the more it slips the greater the pressure to add more features into the next release...