Answer: Perhaps your sources are suggesting that such a barrier will prevent tree roots from working their way up into the improved soil. In my experience that technique doesn't work very well over time, so I would not bother with it. Instead, I would try to pick a site away from trees!
Plastic will also impede drainage and air movement through the soil, and limit the depth of root penetration, so I disagree with your other sources. I do recommend removing pernicious weeds that might make their way through all that soil to plague you later. Just remove any weeds before filling the bed.
At the very least you will want to turn under any sod so there is not an abrupt change from the improved soil to the native soil. This is to help any deep-rooted plants attempting to reach into the native soil.
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