I use a topbox almost exclusively - it's great for the commute, and I rarely use the panniers as I don't need them. And it houses extra brake lights.
However, I must be missing something, everyone who seems to tour (like AMC etc.) uses just panniers and no topbox.
Why? Is it purely aesthetics, or is there a decent reason (aerodynamics?) that precludes it's use? (Bearing in mind there would be almost no weight in it - glasses,gloves, disc lock, wallet sort of thing as I understand that CoG would be adversely affected if loaded.)
If I tour, should I dump the Dropbox and use a waterproof bag if needed? (I can probably cope with just panniers) but the topbox seems handy and I can't say I've noticed any issues so far. What about if I'm carrying more speed than usual?
A tank bag would get around the need for a top box probably.
Looking forward to your thoughts.