Isometrics can be used to increase maximal strength, as well as speed strength (quick contraction at the beginning of range of motion). They create strength within 15 to 20 degrees either way (I believe) of where the contraction took place. If you do not want to decrease speed, try not to contract fully for more than 5-6 seconds. I agree, you may not get much hypertrophy (muscle growth) from isometrics, but strength is more a function of the nervous system. Strength increases when the nervous system can activate more motor units simultaneously. For more details on these things, read Never Gymless by Ross Enamait.
RossTraining - Never Gymless