Every BJJ Black Belt World Champion that I know does some form of strength training.
Two guys of equal skill level go at it, the stronger better conditioned one will win.
Strength training should not be at the expense of your technique training but it is something worth doing even just to help avoid injuries.
Now for me, lifting weights is an effective method of strength training (and I enjoy it). Other people use other methods for various reasons but the toughest guys I have been on the mats with all lift heavy iron.