Why is this a poll thread? Of course someone who doesn't really know / play LC will say 'Omg test everything'. It's very irrelevant imo. Anyway, I think since Berry Juice has an actual fact-based argument to be 'banned before testing', I don't see why it's a bad idea.
Because it can't hurt and there's no real rush? Not like the next Pokemon games are going to be coming any time soon--at the
very least, there will be another year before they come out. I don't need to play Little Cup to understand that much.
Plus, right now, we don't know how it will actually affect the metagame. We can very safely say that it will be a very potent force, but whether it will actually hit the line of being broken and just how it will actually affect the metagame is hard to say, especially since every Pokemon, and not just one or some, can hold Berry Juice.
Thus, even if it seems potentially obvious, there's no harm in a test to at least make sure that that's actually the case, and that the theorymon holds true. There's no need to rush, so a test is find. And if it does obviously turn out to be broken, ala Deoxys-S in stage 3, it can quickly be removed in the same way, but at least we'll have that actual test to back it up and not just some theorymonning.