Thats why I only do it on slippery surfaces. I find a locked brake helps with stability, sometimes the rear wheel will spin just from whe weight of you on the pedals. if it's a slippy off camber / gnarly rock it also helps resist the front wheel turning (i.e if the top of the rock is on a slope, front wheel wants to turn if all your weight is hanging off it). As the rock is slippery, the tyre slides on it as you lift up and doesn't affect the move.
Anyway, personal preference I suppose. I only really ride natural so have to find these little techniques that help me out, and it's very specific to the type of rock you're on.