You would need to boil the mash for it to be concidered decoction, otherwise you're just doing a step mash. I don't think decoctions are really needed with most of the malts today. But, if you're doing a style that is traditionally decocted, then you should include it to keep with style guidelines. It does add a flavor that can't really be matched in my opinion. The closest you can get (in my experience) is by adding 20L crystal and not doing the decoction.
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Another HERMS rig...