Boomkickers are useful when there is light wind. It will overcome the weight of the boom, holding it up, allowing the main to twist off properly and trim better. They can also hold the boom up when dousing the main.
A rigid vang should be spring loaded and sized for your boat. The spring will hold the boom up as a Boomkicker or topping lift. The rigid vang would need extra purchase to overcome the spring, and hold the boom down as needed in higher wind.
All this can be accomplished using an easily adjustable topping lift. I lead my topping lift forward on the boom, so I can adjust it even if the boom is out over the water. Some people lead the topping lift back down from the top of the mast, but this incurs more weight aloft due to a block at masthead, and double the length of line to come back down the mast. One of the drawbacks to a topping lift is when it gets caught on the leech of the main.
I dunno, I guess a Boomkicker might be a more cost effective addition for someone who already has a good 4:1 "soft" vang, than replacing the soft vang with a rigid vang.
But, yeah, you need a vang to control boom height when off the wind. Close hauled, a proper and easily adjustable mainsheet traveller does more to control sail shape and angle of attack.
I couldn't sail my traveller-less boat without vang-sheeting using the adjustable topping lift in combo with the soft vang.