That's not true in all RTSes. Take StarCraft, for example, and there are plenty of games on record that were decided not just by 1 unit, but by 1 attack from 1 unit. There are Zerg players, for example, who have developed a reputation for creating havoc after getting a single zergling (the smallest and cheapest attacking unit) into their opponent's base. A single shot from a Protoss reaver can mean the difference between taking minimal damage and losing half of your workers (and subsequently the game).