It's the same as me putting jack stands under the frame. In my opinion they do work BUT don't crank them up to the point that they lift the trailer. You want them just SNUGGED up thus not causing any twists in the frame.
Here's how I
stabilize my rig.
If memory serves me, sreigle has added just such scissor jacks on his rig. Maybe he'll jump in here and give his side of the story.
Also like foggyb I only use my jack stands when on an extended stay of a few weeks or more. Maybe if they were permanently mounted scissor jacks I'd use them more often.