For example, I played in a fun pseudo-history game a few years back set in Bucharest, Romania in the 1300s. The city was fun to explore and we found some ancient tunnels crisscrossing beneath the city. It was a pretty standard dungeon exploration adventure with a city above. It was too large to really discover it all (a big megadungeon), but we were having fun.
Unfortunately, the DM was running a plotline and not a standard game, so we were supposed to head North into what is present-day Ukraine and then Moscow. Driving us there was the invading Ottoman Empire (goblinoids). Rumors had much of the country running North and West (to Italy, the other option) and if we didn't go too, we would be invaded, possibly killed, and conquered along with all the others who did not leave.
Being heroes, someone came up with the brilliant idea of staging a resistance beneath the city (Vive la Resistance!). We would explore the dungeon below the city, while using strike attacks on the army above to hopefully disrupt their advance. It was a great idea, only the DM nixed it by breaking character and telling us to go to the Ukraine or Italy. There was no other choice.
Did he have the materials for the dungeon below? Yes. Could he have devised an overtaking of the city and our own preparations beforehand of a resistance? I think so. But that wasn't the Campaign Story... and we went with his idea and not our own. Soon after I left the group.
The idea of a freeform campaign isn't impossible. It's about allowing the players to choose their own fate, not forcing one upon them. Training players they should wait for the DM story or hook is frustrating for those of us who desire players who'll be heroes. Those who make their lives instead of letting it happen to them. I think those are the stories that get told afterwards; the stories of brilliant ideas like the one above (if it had come to fruition) and not some writer's predesigned (and predestined) plotline.
EDIT: If 6 heads are better than 1, why use only one person's ideas?