In the Marineford arc, all Marines were present in the battlefield, except for the Gorosei. They knew that the Whitebeard Pirates would come to attack. So why didn't the Gorosei join the battle?
The Gorosei are the heads of the World Government and the rulers of the entire world.
Ace's execution was to be held at Marineford, in the presence of the three Admirals (aka the powerhouses), the legendary Monkey D Garp, several Vice Admirals, lots of Captains and Commanders, the Shichibukai and the Fleet Admiral Sengoku himself. This would make up atleast majority of the Navy, if not entire.
From this roll call it's obvious that the Gorosei wanted to get the job done quickly without their further meddling. Also, with forces like Whitebeard and his allies, going into the action could prove highly risky, because of their status.
Moreover, Oda sensei is building up more suspense and story to how the Gorosei will make a huge impact to the series. Marineford Arc might have been too small to involve the elements of the Gorosei. Also, their strength introduction would mean a history recall, which would need to be linked with the current storyline.
It's important to make a distinction between the Navy and the World Government. The Navy is a branch of the World Government, but they are not one and the same. The Gorosei rule the world and although I think it perfectly possible that they are strong in combat, it is the job of the Navy to fight. You will also notice Cipher Pol was not in Marineford as this is another branch of the World Government.