"We have X number of patents in our pool that we say WebM infringes on; pay us and we won't sue you." It'll practically negate the entire purpose of using WebM instead of H.264.
Edit: Until a court of law has ruled that WebM infringes on patents held by MPEGLA, what they're doing is quite literally the definition of Fear, Uncertainty, and Doubt. They want you to Fear a lawsuit, because you're Uncertain about the legitimacy of their claims, and Doubt that you could either afford the costs or even defeat their claims. You can't possibly tell me that FUD is inappropriate for this situation.