In dressage, its more of a foot rest for me.
However, in jumping, you HAVE to have weight in your stirrups. Where else are you going to place it over the jump? I disagree that is causes you to lock your ankles. You UNLOCK your ankles, then the weight flows down into your heel and it absorbs the shock so that you don't bounce.
I also put a little more pressure in the stirrups while posting. If I didn't, I'd probably compensate by gripping with my knee.
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman