I do not add weights because for me it is not worth the effort. In the past when plugs were not properly balanced for casting distance it would have helped, but now adding weights to plugs like the one you have posted in the picture there will cut short the casting distance.
The plug in your picture has a magnet where the front hook is, acting as
1) Center of Gravity
2) To attract the floating bearing in the lure
When you cast the bearing detaches from the magnet and transfers the weight to the end of the lure so that the tail of the lure will always be ahead and land first. This ensures that the lure flies through the air (without wobble) straight as an arrow and smoothly giving you good distance. When it hits the water the bearing is pulled back to the magnet and sticks to it, transferring its weight to the center of the lure offering excellent balance in water.
My advice would be to buy the right plugs.
Weights would help if you want to improve your lures diving ability and go deeper.