First, I'd say regarding your calf muscles that they already receive alot of attention (think of your sprinting workouts) and it isn't absolutely necessary to hit them directly in the wt. room. I believe the gastroc muscles indirectly get trained during a handful of other lower body exercises as well.
In regards to leg curls and knee extensions, I'm just not a 'fan' of them because of the position these lifts place the knees in throughout the whole movement. The knees are 'locked' in or set in a position that does not allow for a large range of motion throughout, which can cause problems (both that I've seen and heard about). Look at those 2 exercises compared to let's say, the back squat. In the squat, the knee joint is free to move throughout a large range of motion. Range of motion is not hampered by a pad or machine.
You CAN do them as an auxiliary exercise, but I would place them at the bottom of your list of lifts to focus on…that's for sure.