If you have a 4 piece bonus that is so bad (both the set bonus and the itemization of the tier pieces themselves) that people are actively gearing around avoiding it, it's a design/balance problem that needs to be fixed.
Blizz has stated many times that they do not want all pieces to be perfectly itemized. They deliberately do this to create "choices."
The thing I don't like about the shaman 4pc has less to do with how much output it provides and more to do with tying a mobility cooldown to a throughput cooldown. Might find yourself saving SWG for high damage phases as an additional burst CD instead of using it to cast on the run.
Maybe the option is good. I dunno. Pretty out of touch with shaman stuff lately.
A 0.3%-0.5% gain from a 4 piece bonus is also against their stated design intent. When you have the combination of that, plus poorly itemized tier pieces, you make the 4 piece bonus an actual downgrade to gear to, because you gain more than 0.5% from swapping bad itemization with good itemization, especially if you get warforged off pieces.
They change/buff set bonuses all the time, because they are terrible or underpowered. They ended up having to change our T14 bonus a couple months into the tier because people were just avoiding it.