They don't cancel each other out. Once I started lifting weights i noticed gains in other areas, for instance, without training cardio for 4 weeks I ran a 22:59 5k. If you're looking to maximize both do Defranco's Westside for skinny bastards.
excessive muscle mass can reduce agility if you'r elike benching like crazy for muscle mass - but you can def incorporate weights into your workouts and build muscle mass and not lose agility - look at kobe bryant, for example
They are not necessarily mutually exclusive. You will need to continue to train agility. The issue is larger muscles have shortened and have reduced potential energy. Basically because they are shortened they do not spring as well.