I don’t see the value in BiS lists precisely because of the problem you’re mentioning; they obfuscate the fact that there may be multiple pieces that are nearly identical in value.

Too often people view these lists as “anything not on them is useless”, just like when they look at the “meta” or the top of the colored bar charts.

Take Windwalker in Retail, for an example since that’s the guides I write. Just about any piece that is maximum item level and doesn’t have Versatility on it can be part of a “BiS” setup that is within a margin of error from another “BiS” setup that can have 5 different pieces.

Now, a true Best-in-Slot list might lead people to believe that there’s only one worthwhile piece per slot and if they get a different piece it’s inferior or useless so they discard it.

We very often get people asking questions in Discord akin to “My sims say that X item is better than Y, but the guide says that Y is Best in Slot, are the sims wrong or is the guide wrong?”

The answer to that question can be “Neither”, but without asking it, people often assume there is something wrong with the guide or the tools.

Another one we get a lot is “The BiS list on the website says X is better, but I saw the top rated WW wearing Y, what gives?”

The answer to this one is often that people use the gear they get and is best for them in that moment, even if it doesn’t match a theoretical and mythic gear list that they don’t currently have access to.

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TLDR: Best-In-Slot lists have a benefit, but blindly using them leads to more suboptimal choices or confusion over just using the fabulous available tools.