A buddy of mine regularly fishes a big worm in deep water (over 20 feet) and uses a large shaky head rather than a Texas rig? He says the shaky "stands the worm up". Truth? I can't imagine it would do any different. I tried it out in my pool and I'm not seeing it.
Think outside the box. A shakey head has a hook shank that is inside a spherical or oblong lead head. A TX rig has a bullet weight slip-type sinker. This is going to give you way different actions and application uses. A TX rig is typically used the flip, pitch, and drag, while a shakey head is primarily used for casting and dragging on the bottom.
Can they be used interchangeably in many situations? Sure.
Can the be used interchangeably in all situations? No.