We needed a top box as were two up on our trip and we needed the space. Personally I had no issues with the topbox on the GS: it had my camera kit it in as well as camping gear. Not the best place, but the camera did not suffer and it was all locked, too. The top box also locked the panniers shut, too.

We had two panniers, a top box, a tank bag and two roll-bags over the panniers. Retrospectively, a ridiculous amount, but when prepping: your minimum is always less than you realise!
This got wittled down over the course of the trip as we realised that we did not need it all, leaving us with one half full roll bag, up over the top box. Not the best place, but the most convenient at the time.

I agree that packing light is a good plan (although you might not believe it when I packed for the trip). If we had not needed the box, it might well have stayed at home. That said, it doubled as a back rest for the other half!

Bottom line is we went on some pretty nasty surfaces and the GS survived, despite weighing over 450kg with riders and luggage. If you can do without, great, but I think you learn to adapt and it is a useful storage area: use if for lighter stuff perhaps, or stuff you use frequently, like sleeping bags, stove and food.

Consider your bike, and consider the frame strength and bike weight to see if the added weight will be a major disturbance to your ride, or a risk to the frame. Top-boxes do not have to be a 50ltr jobby: they can be small, too.

If you DO need it, just ride accordingly: it will not make your journey impossible.

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Adventure: it's an experience, not a style!
(so ride what you like, but ride it somewhere new!)