air cushion/water hammer arrestors in plumbing

How important is it to have air cushions and/or water hammer arrestors in plumbing? My current galvanized plumbing has some sections of pipe where there is a long straight section coming off a "T" that simply ends in a cap. My assumption is that is an air cushion for preventing water hammer. Is something like this normally necessary if you do not have excessively high water pressure? Is something like this valuable is catching dirt, sediments, whatever in the pipes and preventing clogging of the fixtures and aerators?
Since I'm going to be installing PEX I'm not terribly concerned with water hammer, but what about putting sediment traps in the lines? Would it a good idea to do this, or are there better ways to filter out city water refuse?
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Eigenvector wrote:

I would sill have them. You need them because the whole stream of water is headed down up and around that pipe towards the end. When you shut it off all that water has a lot of momentum and it needs that cushion. The PEX might take care of it, or might not, why chance it, putting it in now will be a lot easier than later.
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Joseph Meehan

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