If he hadn't lost his ship in a game of cards to Han Solo, the Rebel Alliance would never have won.
Oh, believe me, Lando Calrissian spent plenty of time thinking about that. After he became the administrator of Bespin, Lando had very little time for himself, but all the time in the world to reflect on just what had become of his life. He seemed to have accomplished the dream of the rogue: going mainstream, having great influence, earning respect, being taken seriously.
But all the time, he kept his eye on that ship. He knew what his old friend was up to at all times. He knew when the ship ran afoul of the Empire. He prided himself on staying out of trouble. But he counted down the hours until the Empire came to Cloud City. It was inevitable. He was ready.
He was nearly ready. He knew he could control the situation, at least to a certain extent. He would have to compromise. He would have to look like the bad guy. But he had no choice. In order to keep the upper hand, he would have to bluff, after all. He would have to lose a few rounds.
But Lando never lost for long.
He knew how to keep track of the players. He never lost focus. He made sure the ship kept flying. He made sure things kept happening. Even when it seemed like everyone else was doing the important work, there was Lando behind them.
Manipulating would be a bit strong, but that's the short of it, really. You can thank him for that. Without Lando Calrissian...Well, that would be a pretty scary prospect, wouldn't it? And thus, could there be anyone more important?
Don't bet on it.