Whether you deem line guards necessary or not will largely come down to experience, the waters you fish and the depth of the arbour of the reel being used. I have some reels with them and some without. I can Wallis cast with a line guard fitted, so that factor isn't an issue for me. Shallow arbours are more likely to be a pain in less than perfect conditions. However, the shallowest centrepins I have don't have line guards. They don't tend to get an outing if the wind is much more than a light breeze.
One of the biggest factors for me sometimes using line guards is that I tend to fish rivers that flow east with a dash of south. The prevailing winds tend to result in a distinctly less than ideal downstream wind coming down the valleys. If I were fishing rivers that Mostly ran west or south, I doubt I'd value line guards much. Experience means that I no longer have to rely on a line guard as I used to. However, as they don't really inhibit me in other ways (Wallis casting), I'm quite happy to leave them fitted to those reels that have them. I can well understand why they offer nothing to some centrepin anglers. However, I do think that some experienced users tend to forget what learning was really like. They can give a beginner a bit more leeway when they are learning.